How to use a Class

Hi,
 I'm new to ASP.NET, and I have a problem with a class. So I created a folder in my ASP.NET project called App_Code. Then I right-clicked and clicked "Add New Item" and selected class. It added a file named class1.cs to my App_Code folder. Ok, So when I try to call the class in the codebehind of my default.aspx.cs file, it says it isn't defined/declared/whatever. What am I missing? 
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