Polymer Electrolyte Membrane (PEM) fuel cells hold great promise as a potential alternative to conventional fossil-fuel-powered energy sources. However, they produce waste heat, and the water used in their system must be carefully controlled and monitored. This monograph in Momentum Press's 'Sustainable Energy Series' offers the most up-to-date research on advanced methods for managing both waste heat and monitoring water flows and water concentration in PEM fuel cells. The most workable solutions to these problems and the challenges that still lie ahead are emphasized. The reader will find: A review of multi-component mass transport phenomena, particularly as they apply to gas flow and hydrogen and air flow Coverage of multi-phase flow and heat transfer characteristics of PEM fuel cells Unique coverage of solid water phase behavior of PEM fuel cells and subfreezing temperature operation