Amp; Symbol In Code

I ran my page through the W3C markup validator, and all's well except one thing. If I have the "&" symbol in my URLs, it says: Quote:

Line 143, column 66: cannot generate system identifier for general entity "cat"
Line 143, column 69: reference not terminated by REFC delimiter
Line 143, column 69: reference to external entity in attribute value

I tried doing "&-amp;" (without the -) and "%26" but even though %26 works for the validator, my ASP script doesn't know it's an amp.