CFAST is a two-zone fire model capable of predicting the environment in a multi-compartment structure subjected to a fire. It calculates the time evolving distribution of smoke and fire gases and the temperature throughout a building during a user-prescribed fire. This report describes the equations which constitute the model, the physical basis for these equations, and an evaluation of the sensitivity and predictive capability of the model. This report is an assessment of the model following the outline set forth in ASTM E1355, Standard Guide for Evaluating the Predictive Capability of Deterministic Fire Models.