[en] The NACT program was developed to determine the more important activation products generated when a particle flux, usually neutrons, is incident on a target material. The code first establishes all those reactions that are kinematically possible. User input activity levels and half lives are used to screen out unimportant nuclides for any particular application. If desired, a time dependent gamma ray spectrum can be generated in 13 energy groups. This document provides a summary of the theory used, an outline of the program flow, the input data format, and three sample runs with selected pages of output