The codes are named "the Pre-processing" codes, because they are designed to pre-process ENDF formatted data, for later, further processing for use in applications. This is a modular set of computer codes, each of which reads and writes evaluated nuclear data in the ENDF format. Each code performs one or more independent operations on the data, as described below. These codes are designed to be computer independent, and are presently operational on every type of computer from large mainframe computer to small personal computers, such as IBM-PC and MAC (OSX). The codes are available from the IAEA Nuclear Data Section, free of charge and can be downloaded from http://www-nds.iaea.org/ndspub/endf/prepro/.