An Environmental Control and Life Support System (ECLSS) functional behavior model has been developed for the Stage 6 assembly of Space Station Freedom (SSF) using RDD-100®. This software tool has been used to generate, both in graphic and text formats, system and subsystem descriptions of the ECLSS functional behavior. Program requirements have been traced to the ECLSS functions and the functions are then allocated to system components. Thus, RDD-100® provides a means of verifying that documented requirements are linked to the actual hardware.The model is termed a behavior model because it defines the specific behavior characteristics and functional properties of the ECLSS. Functional elements are graphically arranged to show operating sequences and relationships and are specified in terms of input items, transformations, output items and performance objectives.This paper describes selected behavior diagrams of the ECLSS for the Stage 6 assembly of Space Station Freedom, and discusses the system design data contained in the model that trace requirements to the actual hardware.