If you are getting the following error when testing an application locally, see the information further down this page for an explanation and solution:
"Could not load file or assembly 'System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified."
This error happens when the application references an AJAX control toolkit file required to run certain procedures, and the AJAX toolkit hasn't been installed.
Download and run the .msi file on the link below to install the AJAX control toolkit and refresh the page on which you were getting the error: