The present invention provides membrane cassettes and stacks thereof which are suitable for a use in a variety of electrochemical applications. The invention further provides membrane cassettes which comprise one or more bipolar plates which have one or two reactant or coolant flow fields consisting of at least one groove in opposing surfaces of the bipolar plate. In certain preferred embodiments, the invention provides cassettes and stacks which are suitable for use in fuel cell applications. Particularly preferred embodiments of the invention include design improvements which enhance the performance and reliability of certain components of the fuel cell stack.