Concerns about the environmental effects of CFCs, electric capacity shortages during periods of peak load, and the substantially increased cost of building new electric power plants have generated renewed and growing interest in absorption heat pumps. The particular features of this heat-powered cycle have made it attractive for both residential and industrial applications. The last decade saw intensive research and development efforts of gas-fired absorption systems for both heating and cooling applications in the USA, Europe and Japan. The lecture will describe the principles of absorption heat pumping, the basic single-effect cycles and several multi-stage systems. Working fluids for absorption systems will be discussed. Applications for absorption systems will be reviewed, including solar- and waste heat- powered absorption chillers for commercial and industrial installations, and gas-fired heat pumps for residential use. The computer code ABSIM, developed for simulation of absorption systems in a flexible and modular form, will be introduced. Examples of simulation results will be given for several cycles, including lithium bromide-water double-, and triple-effect cycles and ammonia-water GAX and branched GAX cycles. Components of the system may be integrated into buildings simulation programs.