One of the most frequently encountered problems in the design and development of life support systems concerns system integration and evaluation. Lockheed Missiles & Space Co. has developed a simulation modelling toolkit to assist life support system designers in conducting system design, integration and performance assessment. This work arose from a recognized need for a flexible, easily-visualized simulation tool to support aerospace life support system design. The simulator supports performance analysis and "what if" studies based on the construction of block diagrams of the systems to be simulated. Libraries of physicochemical, bioregenerative and hybrid technologies have been developed. These subroutines can be accessed for use in developing system block diagrams. The blocks specified in a particular model can be easily interchanged with other blocks which perform similar functions to assess changes in system performance. The simulation tool supports plotting of user-selectable model outputs to one or more on-screen strip chart recorders, each having up to four channels. This paper describes key features of several blocks, as well as a hybrid physicochemical/bioregenerative life support system model.