diff --git a/SurveyLib.Core/Services/IAnswerService.cs b/SurveyLib.Core/Services/IAnswerService.cs
new file mode 100644
index 0000000..d2122dc
--- /dev/null
+++ b/SurveyLib.Core/Services/IAnswerService.cs
@@ -0,0 +1,12 @@
+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface IAnswerService
+{
+    Task<bool> AddAnswerAsync(Answer answer);
+    Task<bool> UpdateAnswerAsync(Answer answer);
+    Task<bool> DeleteAnswerAsync(int id);
+    Task<IEnumerable<Answer>> GetAnswersByCompletionIdAsync(int completionId);
+    Task<IEnumerable<Answer>> GetAnswersByQuestionIdAsync(int questionId);
+}
\ No newline at end of file
diff --git a/SurveyLib.Core/Services/ICompletionService.cs b/SurveyLib.Core/Services/ICompletionService.cs
new file mode 100644
index 0000000..3a9430d
--- /dev/null
+++ b/SurveyLib.Core/Services/ICompletionService.cs
@@ -0,0 +1,11 @@
+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface ICompletionService
+{
+    Task<bool> AddCompletionAsync(Completion completion);
+    Task<bool> UpdateCompletionAsync(Completion completion);
+    Task<bool> DeleteCompletionAsync(int id);
+    Task<IEnumerable<Completion>> GetCompletionsBySurveyIdAsync(int surveyId);
+}
\ No newline at end of file
diff --git a/SurveyLib.Core/Services/IQuestionService.cs b/SurveyLib.Core/Services/IQuestionService.cs
new file mode 100644
index 0000000..af0d6d5
--- /dev/null
+++ b/SurveyLib.Core/Services/IQuestionService.cs
@@ -0,0 +1,11 @@
+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface IQuestionService
+{
+    Task<bool> AddQuestionAsync(QuestionBase question);
+    Task<bool> UpdateQuestionAsync(QuestionBase question);
+    Task<bool> DeleteQuestionAsync(int id);
+    Task<IEnumerable<QuestionBase>> GetQuestionsBySurveyIdAsync(int surveyId);
+}
\ No newline at end of file
diff --git a/SurveyLib.Core/Services/ISurveyService.cs b/SurveyLib.Core/Services/ISurveyService.cs
new file mode 100644
index 0000000..6ec6d55
--- /dev/null
+++ b/SurveyLib.Core/Services/ISurveyService.cs
@@ -0,0 +1,14 @@
+using SurveyLib.Core.Models;
+
+namespace SurveyLib.Core.Services;
+
+public interface ISurveyService
+{
+    Task<bool> AddSurveyAsync(Survey survey);
+    Task<bool> UpdateSurveyAsync(Survey survey);
+    Task<bool> DeleteSurveyAsync(int id);
+    Task<IEnumerable<Survey>> GetSurveysAsync();
+    Task<Survey> GetSurveyByIdAsync(int id);
+    Task<Survey> GetSurveyWithQuestionsByIdAsync(int id);
+    Task<Survey> GetSurveyWithAnswersByIdAsync(int id);
+}
\ No newline at end of file