 <p>They are ASP.NET Expressions.</p> <p>A very common usage is when using a resource file, like so</p> <pre><code>&lt;asp:Label id="label1" runat="server" text="&lt;%$ Resources: Messages, ThankYouLabel %&gt;" /&gt; </code></pre> <p>Read more on them here: <a href="http://msdn.microsoft.com/en-us/library/d5bd1tad.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/d5bd1tad.aspx</a></p>