The desired performance of a polymer electrolyte fuel cell is always hindered by mismatching of operating conditions (such as temperature, flow rates of fuel and oxidant, level of humidification of reactants, etc.) with parameters associated with the cell performance (such as electrolyte conductivity, net back diffusion of water behavior in the membrane electrolyte assembly, etc.). This paper presents some design guidelines for polymer electrolyte fuel cells by establishing relationships among critical parameters of the fuel cell and is particularly useful for the developmental stage of mini/microfuel cells operating under nonpressurized condition and without external humidification of fuel. (c) 2007 The Electrochemical Society.