Proton exchange membrane (PEM) fuel cells are considered one of the best potential alternative power sources for automobiles. For this application, high efficiency and high power density are required. Pressurizing the fuel cell system can give higher efficiency, higher power density and better water balance characteristics for the fuel cell, but pressurization uses a percentage of the fuel cell output power. The compressor used to elevate the pressure has a direct effect on the system efficiency and water balance characteristics. A variety of compressors are being developed for fuel cell applications. Two compressor and expander technologies are discussed in this paper: the Opcon 1050 positive displacement twin-screw compressor and expander, and a Honeywell turbocompressor and expander. The effect of these compressors and expanders on the system at maximum load, low load, and set minimum airflow are examined. The effects of ambient conditions, stack temperature, and increased twin-screw compressor pressure are also examined. The turbocompressor proves to be a superior machine in terms of efficiency, and therefore offers the most promising effect on system efficiency of the two compressors. The twin-screw compressor, on the other hand, offers more flexible pressure ratio and better water balance characteristics at low fuel cell loads, which is an important factor with PEM fuel cell systems. Increased ambient and stack temperature has a significant negative effect on the water balance and a small positive effect on efficiency. Increasing the pressure for the twin-screw compressor significantly improves the water balance characteristics with some loss in efficiency. These results show the importance of determining the system operating range and operating conditions in the choice of a compressor for a fuel cell system / Master of Science