Getting the HTTP Referrer in ASP.NET

You could use the UrlReferrer property of the current request:

Request.UrlReferrer

This will read the Referer HTTP header from the request which may or may not be supplied by the client (user agent).

Request.Headers["Referer"]

Explanation

The Request.UrlReferrer property will throw a System.UriFormatException if the referer HTTP header is malformed (which can happen since it is not usually under your control).

Therefore, the Request.UrlReferrer property is not 100% reliable - it may contain data that cannot be parsed into a Uri class. To ensure the value is always readable, use Request.Headers["Referer"] instead.

As for using Request.ServerVariables as others here have suggested, per MSDN:

Request.ServerVariables Collection

The ServerVariables collection retrieves the values of predetermined environment variables and request header information.

Request.Headers Property

Gets a collection of HTTP headers.

Request.Headers is a better choice than Request.ServerVariables, since Request.ServerVariables contains all of the environment variables as well as the headers, where Request.Headers is a much shorter list that only contains the headers.

So the most reliable solution is to use the Request.Headers collection to read the value directly. Do heed Microsoft's warnings about HTML encoding the value if you are going to display it on a form, though.