diff --git a/agiledancers-be/pom.xml b/agiledancers-be/pom.xml
index 477cbba..26611a6 100644
--- a/agiledancers-be/pom.xml
+++ b/agiledancers-be/pom.xml
@@ -18,6 +18,19 @@
             <groupId>com.google.ortools</groupId>
             <artifactId>ortools-java</artifactId>
         </dependency>
+
+        <!-- test dependencies -->
+        <dependency>
+            <groupId>net.datafaker</groupId>
+            <artifactId>datafaker</artifactId>
+            <scope>test</scope>
+        </dependency>
+
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
 </project>
\ No newline at end of file
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/Main.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/Main.java
deleted file mode 100644
index 00ddbb4..0000000
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/Main.java
+++ /dev/null
@@ -1,17 +0,0 @@
-package nl.agilecc.agiledancers;
-
-//TIP To <b>Run</b> code, press <shortcut actionId="Run"/> or
-// click the <icon src="AllIcons.Actions.Execute"/> icon in the gutter.
-public class Main {
-    public static void main(String[] args) {
-        //TIP Press <shortcut actionId="ShowIntentionActions"/> with your caret at the highlighted text
-        // to see how IntelliJ IDEA suggests fixing it.
-        System.out.printf("Hello and welcome!");
-
-        for (int i = 1; i <= 5; i++) {
-            //TIP Press <shortcut actionId="Debug"/> to start debugging your code. We have set one <icon src="AllIcons.Debugger.Db_set_breakpoint"/> breakpoint
-            // for you, but you can always add more by pressing <shortcut actionId="ToggleLineBreakpoint"/>.
-            System.out.println("i = " + i);
-        }
-    }
-}
\ No newline at end of file
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/AbstractEntity.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/AbstractEntity.java
new file mode 100644
index 0000000..3a08a0e
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/AbstractEntity.java
@@ -0,0 +1,30 @@
+package nl.agilecc.agiledancers.model;
+
+import lombok.*;
+
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Set;
+
+@Getter
+@Setter
+@EqualsAndHashCode
+@ToString
+@NoArgsConstructor
+public abstract class AbstractEntity {
+    private Long id;
+
+    protected <T> void addNullSafe(Set<T> set, T t) {
+        emptyIfNull(set).add(t);
+    }
+
+    private <T> Set<T> emptyIfNull(Set<T> set) {
+        return set == null ?
+                new HashSet<>() :
+                set;
+    }
+
+    protected <T> Set<T> unmodifiableNullSafe(Set<T> set) {
+        return set == null ? Collections.emptySet() : Collections.unmodifiableSet(set);
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Coach.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Coach.java
index f008c3c..326b14c 100644
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Coach.java
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Coach.java
@@ -1,6 +1,12 @@
 package nl.agilecc.agiledancers.model;
 
-public class Coach {
-    private Long id;
-    private String name;
+import lombok.*;
+
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class Coach extends AbstractEntity {
+    @ToString.Include
+    private final String name;
 }
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceGroup.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceGroup.java
index 5914190..c81a203 100644
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceGroup.java
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceGroup.java
@@ -1,17 +1,46 @@
 package nl.agilecc.agiledancers.model;
 
+import lombok.*;
 import nl.agilecc.agiledancers.model.specifications.DanceGroupStyle;
 import nl.agilecc.agiledancers.model.specifications.DanceGroupType;
 
+import java.util.Collections;
+import java.util.HashSet;
 import java.util.Set;
 
-public class DanceGroup {
-    private Long id;
-    private String category;
-    private DanceGroupType type;
-    private int startAge;
-    private int endAge;
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class DanceGroup extends AbstractEntity {
+    @ToString.Include
+    private final String category;
+    @ToString.Include
+    private final DanceGroupType type;
+    @ToString.Include
+    private final int startAge;
+    @ToString.Include
+    private final int endAge;
+    @ToString.Include
     private Set<Dancer> dancers;
+    @ToString.Include
     private Set<DanceGroupStyle> styles;
+    @ToString.Include
     private Coach coach;
+
+    public Set<Dancer> getDancers() {
+        return super.unmodifiableNullSafe(dancers);
+    }
+
+    public void addDancer(Dancer dancer) {
+        super.addNullSafe(dancers, dancer);
+    }
+
+    public Set<DanceGroupStyle> getStyles() {
+        return super.unmodifiableNullSafe(styles);
+    }
+
+    public void addStyle(DanceGroupStyle style) {
+        super.addNullSafe(styles, style);
+    }
 }
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceSchool.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceSchool.java
new file mode 100644
index 0000000..63803d7
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DanceSchool.java
@@ -0,0 +1,50 @@
+package nl.agilecc.agiledancers.model;
+
+import lombok.*;
+
+import java.util.Collections;
+import java.util.Set;
+
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class DanceSchool extends AbstractEntity {
+    private final String name;
+    private final Set<Dancer> dancers;
+    private final Set<DanceGroup> danceGroups;
+    private final Set<Show> shows;
+    private final Set<Coach> coaches;
+
+    public Set<Dancer> getDancers() {
+        return super.unmodifiableNullSafe(dancers);
+    }
+
+    public Set<DanceGroup> getDanceGroups() {
+        return super.unmodifiableNullSafe(danceGroups);
+    }
+
+    public Set<Show> getShows() {
+        return super.unmodifiableNullSafe(shows);
+    }
+
+    public Set<Coach> getCoaches() {
+        return super.unmodifiableNullSafe(coaches);
+    }
+
+    public void addDancer(Dancer dancer) {
+        super.addNullSafe(dancers, dancer);
+    }
+
+    public void addDanceGroup(DanceGroup danceGroup) {
+        super.addNullSafe(danceGroups, danceGroup);
+    }
+
+    public void addShow(Show show) {
+        super.addNullSafe(shows, show);
+    }
+
+    public void addCoach(Coach coach) {
+        super.addNullSafe(coaches, coach);
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Dancer.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Dancer.java
index 5b3dfec..a628a44 100644
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Dancer.java
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Dancer.java
@@ -1,7 +1,21 @@
 package nl.agilecc.agiledancers.model;
 
-public class Dancer {
-    private Long id;
-    private String name;
-    private int age;
+import lombok.*;
+
+import java.time.LocalDate;
+import java.time.Period;
+
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class Dancer extends AbstractEntity {
+    @ToString.Include
+    private final String name;
+    @ToString.Include
+    private LocalDate birthDate;
+
+    public int age() {
+        return Period.between(birthDate, LocalDate.now()).getYears();
+    }
 }
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DayPart.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DayPart.java
index ba068f5..6503d82 100644
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DayPart.java
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/DayPart.java
@@ -1,12 +1,17 @@
 package nl.agilecc.agiledancers.model;
 
+import lombok.*;
 import nl.agilecc.agiledancers.model.specifications.DayPartType;
 
 import java.util.List;
 
-public class DayPart {
-    private Long id;
-
-    private DayPartType type;
-    private List<DanceGroup> danceGroups;
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class DayPart extends AbstractEntity {
+    @ToString.Include
+    private final DayPartType type;
+    @ToString.Include
+    private final List<DanceGroup> danceGroups;
 }
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Show.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Show.java
index b23a772..de61f10 100644
--- a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Show.java
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/model/Show.java
@@ -1,8 +1,14 @@
 package nl.agilecc.agiledancers.model;
 
+import lombok.*;
+
 import java.util.List;
 
-public class Show {
-    private Long id;
-    private List<DayPart> dayParts;
+@Getter
+@AllArgsConstructor(access = AccessLevel.PRIVATE)
+@Builder
+@ToString(onlyExplicitlyIncluded = true)
+public class Show extends AbstractEntity {
+    @ToString.Include
+    private final List<DayPart> dayParts;
 }
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/AbstractRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/AbstractRepository.java
new file mode 100644
index 0000000..0468b13
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/AbstractRepository.java
@@ -0,0 +1,27 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.AbstractEntity;
+
+import java.util.*;
+
+public abstract class AbstractRepository<T extends AbstractEntity> {
+    private long sequence = 0L;
+
+    protected Map<Long, T> entities = new LinkedHashMap<>();
+
+    public T add(T entity) {
+        if (entity.getId() == null) {
+            entity.setId(sequence++);
+        }
+        entities.put(entity.getId(), entity);
+        return entity;
+    }
+
+    public List<T> findAll() {
+        return List.copyOf(entities.values());
+    }
+
+    public T findById(long id) {
+        return entities.get(id);
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/CoachRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/CoachRepository.java
new file mode 100644
index 0000000..2257e5c
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/CoachRepository.java
@@ -0,0 +1,7 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.Coach;
+
+public class CoachRepository extends AbstractRepository<Coach> {
+
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceGroupRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceGroupRepository.java
new file mode 100644
index 0000000..57f4ed2
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceGroupRepository.java
@@ -0,0 +1,6 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.DanceGroup;
+
+public class DanceGroupRepository extends AbstractRepository<DanceGroup> {
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceSchoolRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceSchoolRepository.java
new file mode 100644
index 0000000..de5ce27
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DanceSchoolRepository.java
@@ -0,0 +1,6 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.DanceSchool;
+
+public class DanceSchoolRepository extends AbstractRepository<DanceSchool> {
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DancerRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DancerRepository.java
new file mode 100644
index 0000000..98cf1f3
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DancerRepository.java
@@ -0,0 +1,6 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.Dancer;
+
+public class DancerRepository extends AbstractRepository<Dancer> {
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DayPartRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DayPartRepository.java
new file mode 100644
index 0000000..5d0a703
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/DayPartRepository.java
@@ -0,0 +1,6 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.DayPart;
+
+public class DayPartRepository extends AbstractRepository<DayPart> {
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/ShowRepository.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/ShowRepository.java
new file mode 100644
index 0000000..4e96e32
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/repository/ShowRepository.java
@@ -0,0 +1,6 @@
+package nl.agilecc.agiledancers.repository;
+
+import nl.agilecc.agiledancers.model.Show;
+
+public class ShowRepository extends AbstractRepository<Show> {
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/CoachService.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/CoachService.java
new file mode 100644
index 0000000..457ae29
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/CoachService.java
@@ -0,0 +1,12 @@
+package nl.agilecc.agiledancers.service;
+
+import nl.agilecc.agiledancers.model.Coach;
+import nl.agilecc.agiledancers.repository.CoachRepository;
+
+public class CoachService {
+    private CoachRepository coachRepository = new CoachRepository();
+
+    public Coach addCoach(String name, Coach coach) {
+        return coachRepository.add(Coach.builder().name(name).build());
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceGroupService.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceGroupService.java
new file mode 100644
index 0000000..a201877
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceGroupService.java
@@ -0,0 +1,27 @@
+package nl.agilecc.agiledancers.service;
+
+import nl.agilecc.agiledancers.model.Coach;
+import nl.agilecc.agiledancers.model.DanceGroup;
+import nl.agilecc.agiledancers.model.Dancer;
+import nl.agilecc.agiledancers.model.specifications.DanceGroupStyle;
+import nl.agilecc.agiledancers.model.specifications.DanceGroupType;
+import nl.agilecc.agiledancers.repository.DanceGroupRepository;
+
+import java.util.Set;
+
+public class DanceGroupService {
+    private final DanceGroupRepository danceGroupRepository = new DanceGroupRepository();
+
+    public DanceGroup add(String category, DanceGroupType type, int startAge, int endAge, Set<Dancer> dancers, Set<DanceGroupStyle> styles, Coach coach) {
+        DanceGroup danceGroup = DanceGroup.builder()
+                .category(category)
+                .type(type)
+                .startAge(startAge)
+                .endAge(endAge)
+                .dancers(dancers)
+                .styles(styles)
+                .coach(coach).build();
+
+        return danceGroupRepository.add(danceGroup);
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceSchoolService.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceSchoolService.java
new file mode 100644
index 0000000..a300d0d
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DanceSchoolService.java
@@ -0,0 +1,29 @@
+package nl.agilecc.agiledancers.service;
+
+import nl.agilecc.agiledancers.model.*;
+import nl.agilecc.agiledancers.repository.DanceSchoolRepository;
+
+public class DanceSchoolService {
+    private final DanceSchoolRepository danceSchoolRepository = new DanceSchoolRepository();
+
+    public DanceSchool add(String name) {
+        DanceSchool danceSchool = DanceSchool.builder().name(name).build();
+        return danceSchoolRepository.add(danceSchool);
+    }
+
+    public void addDancer(Long danceSchoolId, Dancer dancer) {
+        danceSchoolRepository.findById(danceSchoolId).addDancer(dancer);
+    }
+
+    public void addDanceGroup(Long danceSchoolId, DanceGroup danceGroup) {
+        danceSchoolRepository.findById(danceSchoolId).addDanceGroup(danceGroup);
+    }
+
+    public void addCoach(Long danceSchoolId, Coach coach) {
+        danceSchoolRepository.findById(danceSchoolId).addCoach(coach);
+    }
+
+    public void addShow(Long danceSchoolId, Show show) {
+        danceSchoolRepository.findById(danceSchoolId).addShow(show);
+    }
+}
diff --git a/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DancerService.java b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DancerService.java
new file mode 100644
index 0000000..7898505
--- /dev/null
+++ b/agiledancers-be/src/main/java/nl/agilecc/agiledancers/service/DancerService.java
@@ -0,0 +1,15 @@
+package nl.agilecc.agiledancers.service;
+
+import nl.agilecc.agiledancers.model.Dancer;
+import nl.agilecc.agiledancers.repository.DancerRepository;
+
+import java.time.LocalDate;
+
+public class DancerService {
+    private final DancerRepository dancerRepository = new DancerRepository();
+
+    public Dancer addDancer(String name, LocalDate birthDate) {
+        Dancer dancer = Dancer.builder().name(name).birthDate(birthDate).build();
+        return dancerRepository.add(dancer);
+    }
+}
diff --git a/agiledancers-be/src/test/java/sit/DanceSchoolSit.java b/agiledancers-be/src/test/java/sit/DanceSchoolSit.java
new file mode 100644
index 0000000..6d09bfa
--- /dev/null
+++ b/agiledancers-be/src/test/java/sit/DanceSchoolSit.java
@@ -0,0 +1,26 @@
+package sit;
+
+import net.datafaker.Faker;
+import nl.agilecc.agiledancers.model.DanceSchool;
+import nl.agilecc.agiledancers.service.DanceSchoolService;
+import org.junit.jupiter.api.Test;
+
+class DanceSchoolSit {
+    Faker faker = new Faker();
+    int numberOfDanceGroups = 42;
+
+    int aantalDansersTm5 = 10;
+    int aantalDansers6tm10 = 30;
+    int aantalDansers11tot14 = 70;
+    int aantalDansers14tm17 = 140;
+    int aantalDansers18Plus = 100;
+    
+    private DanceSchoolService danceSchoolService = new DanceSchoolService();
+
+    @Test
+    void testDanceSchoolModel() {
+        DanceSchool danceSchool = danceSchoolService.add("24 Dance Studio");
+
+
+    }
+}
diff --git a/pom.xml b/pom.xml
index eb10361..49e5777 100644
--- a/pom.xml
+++ b/pom.xml
@@ -26,6 +26,10 @@
 
         <ortools-java.version>9.12.4544</ortools-java.version>
         <lombok.version>1.18.38</lombok.version>
+
+        <!-- test dependencies -->
+        <datafaker.version>2.4.3</datafaker.version>
+        <junit-jupiter.version>5.12.2</junit-jupiter.version>
     </properties>
 
     <!-- dependencies that can be included by child-project - without stating the version -->
@@ -36,6 +40,21 @@
                 <artifactId>ortools-java</artifactId>
                 <version>${ortools-java.version}</version>
             </dependency>
+
+            <!-- test dependencies -->
+            <dependency>
+                <groupId>net.datafaker</groupId>
+                <artifactId>datafaker</artifactId>
+                <version>${datafaker.version}</version>
+                <scope>test</scope>
+            </dependency>
+
+            <dependency>
+                <groupId>org.junit.jupiter</groupId>
+                <artifactId>junit-jupiter</artifactId>
+                <version>${junit-jupiter.version}</version>
+                <scope>test</scope>
+            </dependency>
         </dependencies>
     </dependencyManagement>