PLEX is a collection of MATLAB routines developed (from 2000–2003) as a research tool for building and studying simplical complexes, generated from real or synthetic point-cloud data. The library defines a new kind of object, called a ’plex’ object, together with various operations (’methods’) on plex objects. According to MATLAB convention, all the code for the object definition and methods are kept in a directory called @plex/, and its parent directory needs to be on the MATLAB search path. PLEX public release 1 (the current version) supports both rational (ie floating point) and mod 2 calculations. The mod 2 routines (as well as one internal PLEX function) are all written in C, as Mex functions in MATLAB. Precompiled binaries and makefiles are provided, in addition to the C source code. PLEX will still run if the Mex functions are not compiled, but mod 2 homology will be unavailable.