diff --git a/3-Domain/src/main/java/category/CategoryPersistenceAdapter.java b/3-Domain/src/main/java/category/CategoryPersistenceAdapter.java
new file mode 100644
index 0000000..75783e8
--- /dev/null
+++ b/3-Domain/src/main/java/category/CategoryPersistenceAdapter.java
@@ -0,0 +1,10 @@
+package category;
+
+import java.util.Set;
+
+public interface CategoryPersistenceAdapter {
+
+	Set<Category> getAll();
+	Boolean add(Category category);
+
+}
diff --git a/3-Domain/src/main/java/category/CategoryRepository.java b/3-Domain/src/main/java/category/CategoryRepository.java
new file mode 100644
index 0000000..dde235f
--- /dev/null
+++ b/3-Domain/src/main/java/category/CategoryRepository.java
@@ -0,0 +1,47 @@
+package category;
+
+import exeptions.CategoryAlreadyExists;
+import exeptions.NonUniqueId;
+
+import java.util.Optional;
+import java.util.Set;
+
+public class CategoryRepository {
+
+	final private CategoryPersistenceAdapter categoryPersistenceAdapter;
+	final private Set<Category> categories;
+
+	public CategoryRepository(CategoryPersistenceAdapter categoryPersistenceAdapter) {
+		this.categoryPersistenceAdapter = categoryPersistenceAdapter;
+		this.categories = categoryPersistenceAdapter.getAll();
+	}
+
+	public Optional<Category> getByName(CategoryName name) {
+		return categories.stream().filter(category -> category.getName().equals(name)).findFirst();
+	}
+
+	public Optional<Category> getById(CategoryId id) {
+		return categories.stream().filter(category -> category.getId().equals(id)).findFirst();
+	}
+
+	public Set<Category> getAll(){
+		return categories;
+	}
+
+	public void add(Category category) {
+		checkDuplicates(category);
+
+		categories.add(category);
+		categoryPersistenceAdapter.add(category);
+	}
+
+	private void checkDuplicates(Category category) {
+		if (this.getByName(category.getName()).isPresent()) {
+			throw new CategoryAlreadyExists("A category with the name " + category.getName() + " already exits.");
+		}
+
+		if (this.getById(category.getId()).isPresent()) {
+			throw new NonUniqueId("A category with the id " + category.getId() + " already exits.");
+		}
+	}
+}
diff --git a/3-Domain/src/main/java/exeptions/CategoryAlreadyExists.java b/3-Domain/src/main/java/exeptions/CategoryAlreadyExists.java
new file mode 100644
index 0000000..614707a
--- /dev/null
+++ b/3-Domain/src/main/java/exeptions/CategoryAlreadyExists.java
@@ -0,0 +1,8 @@
+package exeptions;
+
+public class CategoryAlreadyExists extends RuntimeException {
+
+	public CategoryAlreadyExists(String message) {
+		super(message);
+	}
+}
diff --git a/3-Domain/src/main/java/exeptions/NonUniqueId.java b/3-Domain/src/main/java/exeptions/NonUniqueId.java
new file mode 100644
index 0000000..e710e05
--- /dev/null
+++ b/3-Domain/src/main/java/exeptions/NonUniqueId.java
@@ -0,0 +1,8 @@
+package exeptions;
+
+public class NonUniqueId extends RuntimeException {
+
+	public NonUniqueId(String message) {
+		super(message);
+	}
+}