diff --git a/src/SocialAgent.Data/Repositories/ISocialDataRepository.cs b/src/SocialAgent.Data/Repositories/ISocialDataRepository.cs
index 986e62a..2e75b21 100644
--- a/src/SocialAgent.Data/Repositories/ISocialDataRepository.cs
+++ b/src/SocialAgent.Data/Repositories/ISocialDataRepository.cs
@@ -21,4 +21,8 @@ public interface ISocialDataRepository
     // Poll State
     Task<PollState?> GetPollStateAsync(string providerId, CancellationToken ct = default);
     Task UpsertPollStateAsync(PollState state, CancellationToken ct = default);
+
+    // Retention
+    Task<int> PurgeOldPostsAsync(DateTimeOffset olderThan, CancellationToken ct = default);
+    Task<int> PurgeOldNotificationsAsync(DateTimeOffset olderThan, CancellationToken ct = default);
 }
diff --git a/src/SocialAgent.Data/Repositories/SocialDataRepository.cs b/src/SocialAgent.Data/Repositories/SocialDataRepository.cs
index af8ec48..5a86a69 100644
--- a/src/SocialAgent.Data/Repositories/SocialDataRepository.cs
+++ b/src/SocialAgent.Data/Repositories/SocialDataRepository.cs
@@ -138,4 +138,18 @@ public async Task UpsertPollStateAsync(PollState state, CancellationToken ct = d
         }
         await db.SaveChangesAsync(ct);
     }
+
+    public async Task<int> PurgeOldPostsAsync(DateTimeOffset olderThan, CancellationToken ct = default)
+    {
+        return await db.Posts
+            .Where(p => p.CreatedAt < olderThan)
+            .ExecuteDeleteAsync(ct);
+    }
+
+    public async Task<int> PurgeOldNotificationsAsync(DateTimeOffset olderThan, CancellationToken ct = default)
+    {
+        return await db.Notifications
+            .Where(n => n.CreatedAt < olderThan)
+            .ExecuteDeleteAsync(ct);
+    }
 }
diff --git a/src/SocialAgent.Host/Program.cs b/src/SocialAgent.Host/Program.cs
index 0235674..81b8974 100644
--- a/src/SocialAgent.Host/Program.cs
+++ b/src/SocialAgent.Host/Program.cs
@@ -45,6 +45,7 @@
 // Background services
 builder.Services.AddHostedService<DatabaseMigrationService>();
 builder.Services.AddHostedService<SocialMediaPollingService>();
+builder.Services.AddHostedService<DataRetentionService>();
 
 // Authentication (API key required in non-Development environments)
 builder.Services.AddApiKeyAuthentication(builder.Configuration);
diff --git a/src/SocialAgent.Host/Services/DataRetentionService.cs b/src/SocialAgent.Host/Services/DataRetentionService.cs
new file mode 100644
index 0000000..3e957e4
--- /dev/null
+++ b/src/SocialAgent.Host/Services/DataRetentionService.cs
@@ -0,0 +1,54 @@
+using SocialAgent.Data.Repositories;
+
+namespace SocialAgent.Host.Services;
+
+public class DataRetentionService(
+    IServiceScopeFactory scopeFactory,
+    ILogger<DataRetentionService> logger,
+    IConfiguration configuration) : BackgroundService
+{
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        var retentionDays = configuration.GetValue("SocialAgent:RetentionDays", 30);
+        var intervalHours = configuration.GetValue("SocialAgent:RetentionCheckIntervalHours", 24);
+
+        logger.LogInformation(
+            "Data retention service starting: {RetentionDays} day retention, checking every {Interval}h",
+            retentionDays, intervalHours);
+
+        // Delay startup to let migrations complete
+        await Task.Delay(TimeSpan.FromMinutes(1), stoppingToken);
+
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try
+            {
+                await PurgeOldDataAsync(retentionDays, stoppingToken);
+            }
+            catch (Exception ex) when (ex is not OperationCanceledException)
+            {
+                logger.LogError(ex, "Error during data retention purge");
+            }
+
+            await Task.Delay(TimeSpan.FromHours(intervalHours), stoppingToken);
+        }
+    }
+
+    private async Task PurgeOldDataAsync(int retentionDays, CancellationToken ct)
+    {
+        var cutoff = DateTimeOffset.UtcNow.AddDays(-retentionDays);
+
+        using var scope = scopeFactory.CreateScope();
+        var repository = scope.ServiceProvider.GetRequiredService<ISocialDataRepository>();
+
+        var postsDeleted = await repository.PurgeOldPostsAsync(cutoff, ct);
+        var notificationsDeleted = await repository.PurgeOldNotificationsAsync(cutoff, ct);
+
+        if (postsDeleted > 0 || notificationsDeleted > 0)
+        {
+            logger.LogInformation(
+                "Data retention purge: deleted {Posts} posts and {Notifications} notifications older than {Days} days",
+                postsDeleted, notificationsDeleted, retentionDays);
+        }
+    }
+}
diff --git a/src/SocialAgent.Host/appsettings.json b/src/SocialAgent.Host/appsettings.json
index 2e181ab..4b69de6 100644
--- a/src/SocialAgent.Host/appsettings.json
+++ b/src/SocialAgent.Host/appsettings.json
@@ -16,6 +16,8 @@
   },
   "SocialAgent": {
     "PollingIntervalMinutes": 5,
+    "RetentionDays": 30,
+    "RetentionCheckIntervalHours": 24,
     "DatabaseProvider": "Sqlite",
     "Providers": {
       "Mastodon": {