The pace at which fuel cell systems are widely adopted by the marketplace will be determined primarily by two factors: (1) the rate at which system cost decreases and (2) the rate at which system reliability increases. This paper describes the field reliability and its improvement through a combination of software and hardware changes of Plug Power's GenSys(TM) fleet of 5 kWe (plus up to 9 kW of thermal energy) proton exchange membrane (PEM) fuel cell systems. Plug Power has shipped more than 300 of these systems to more than 50 customer locations in more than 10 countries. This fleet is of sufficient size, and has been operating for a sufficient length of time, to develop statistically significant observations of system reliability. Nondimensionalized probability plots of PEM stack lifetime in field units are presented, and a series of system-level changes are described that have increased PEM stack life by about a factor of 4. Nondimensionalized, system-level reliability statistics are also presented for the installed fleet. Pareto charts describing the top causes for system failures in the field are shown, and the general methodologies for improving system-level reliability are discussed. (c) 2005 Elsevier B.V. All rights reserved.