diff --git a/src/Fdc3.AppDirectory/AppDirectoryException.cs b/src/Fdc3.AppDirectory/AppDirectoryException.cs
new file mode 100644
index 0000000..e3435c1
--- /dev/null
+++ b/src/Fdc3.AppDirectory/AppDirectoryException.cs
@@ -0,0 +1,30 @@
+/*
+ * Morgan Stanley makes this available to you under the Apache License,
+ * Version 2.0 (the "License"). You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * See the NOTICE file distributed with this work for additional information
+ * regarding copyright ownership. Unless required by applicable law or agreed
+ * to in writing, software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ */
+
+using System;
+using System.Runtime.Serialization;
+
+namespace MorganStanley.Fdc3.AppDirectory
+{
+    /// <summary>
+    /// Base class for exceptions thrown by <see cref="IAppDirectory"/> implementations
+    /// </summary>
+    public class AppDirectoryException : Exception
+    {
+        public AppDirectoryException() { }
+        protected AppDirectoryException(SerializationInfo info, StreamingContext context) : base(info, context) { }
+        public AppDirectoryException(string message) : base(message) { }
+        public AppDirectoryException(string message, Exception innerException) : base(message, innerException) { }
+    }
+}
diff --git a/src/Fdc3.AppDirectory/AppNotFoundException.cs b/src/Fdc3.AppDirectory/AppNotFoundException.cs
new file mode 100644
index 0000000..bb6ead2
--- /dev/null
+++ b/src/Fdc3.AppDirectory/AppNotFoundException.cs
@@ -0,0 +1,31 @@
+/*
+ * Morgan Stanley makes this available to you under the Apache License,
+ * Version 2.0 (the "License"). You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0.
+ *
+ * See the NOTICE file distributed with this work for additional information
+ * regarding copyright ownership. Unless required by applicable law or agreed
+ * to in writing, software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions
+ * and limitations under the License.
+ */
+
+using System.Runtime.Serialization;
+
+namespace MorganStanley.Fdc3.AppDirectory
+{
+    /// <summary>
+    /// The exception that is thrown by <see cref="IAppDirectory.GetApp"/> when the app is not found.
+    /// </summary>
+    public class AppNotFoundException : AppDirectoryException
+    {
+        public AppNotFoundException(string appId) : base($"The app with id '{appId}' was not found in the App Directory")
+        {
+        }
+
+        public AppNotFoundException() : base("The app was not found in the App Directory") { }
+        protected AppNotFoundException(SerializationInfo info, StreamingContext context) : base(info, context) { }
+    }
+}
diff --git a/src/Fdc3.AppDirectory/IAppDirectory.cs b/src/Fdc3.AppDirectory/IAppDirectory.cs
index b0f8e06..391ac63 100644
--- a/src/Fdc3.AppDirectory/IAppDirectory.cs
+++ b/src/Fdc3.AppDirectory/IAppDirectory.cs
@@ -33,6 +33,7 @@ public interface IAppDirectory
         /// </summary>
         /// <param name="appId">Application identifier</param>
         /// <returns>The application</returns>
-        Task<Fdc3App?> GetApp(string appId);
+        /// <exception cref="AppNotFoundException">The app was not found</exception>
+        Task<Fdc3App> GetApp(string appId);
     }
 }
diff --git a/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/MorganStanley.Fdc3.AppDirectory.Tests.csproj b/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/MorganStanley.Fdc3.AppDirectory.Tests.csproj
index 993b39d..3ddbcb8 100644
--- a/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/MorganStanley.Fdc3.AppDirectory.Tests.csproj
+++ b/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/MorganStanley.Fdc3.AppDirectory.Tests.csproj
@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
diff --git a/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/Usings.cs b/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/Usings.cs
index 89c3803..b1feab6 100644
--- a/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/Usings.cs
+++ b/src/Tests/MorganStanley.Fdc3.AppDirectory.Tests/Usings.cs
@@ -11,4 +11,4 @@
  * or implied. See the License for the specific language governing permissions
  * and limitations under the License.
  */
-global using Xunit;
\ No newline at end of file
+global using Xunit;