diff --git a/csharp_fundamentals/7_building_types.md b/csharp_fundamentals/7_building_types.md
index ccb5da3..f8a0d78 100644
--- a/csharp_fundamentals/7_building_types.md
+++ b/csharp_fundamentals/7_building_types.md
@@ -169,4 +169,15 @@ A Multicast Delegate is a delegate that holds the references of more than one fu
 * use += operator to chain delegates together and -= operator to remove.
 * A multicast delegate invokes the methods in the invocation list, in the same order in which they are added.
 
-Overall, delegate enable us to declare a method that used like a variable.
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+Overall, delegate enable us to declare a method that used like a variable.
+
+## Defining an Event & Subscribing to an Event
+
+Reference:
+* [event (C# reference)](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/event)
+
+Using delegate, we can implement Event Handler in C# to achieve subscription.
+
+Event is not popular within ASP.NET, but very popular in desktop app & Mobile development (e.g. mouse clicking, keyboard pressing)
+
+TODO: research this part. It's very complicated.
\ No newline at end of file