This file is intended to explain how to install the Cygnus port of the gcc compiler so that it can be used to compile mex files within MATLAB 5 for windows. It is written for the b18 version of that compiler. In fact b19 has just been released, and I will update this file when we have some experience of the new release. There are two main advantages to using the Cygnus gcc over other c compilers. The first is that it free. The second is that the Cygnus environment emulates a considerable degree of Unix functionality within 32 bit Windows, so you may well be able to use the same c mex source files for matlab on Unix and NT/Windows 95.