This book will cover one of the most important aspects of fuel cell research and development, fuel cell durability. The rather broad topic of fuel cell durability will be covered from different viewpoints. First, the durability and degradation issues of catalyst materials (both anode and cathode catalysts) will be described in individual contributions as well as stability aspects from carbon support materials. A following chapter is completely dedicated to important topics from membrane materials, i.e., chemical and physical degradation as well durability from newly developed hydrocarbon membranes. After discussion of stability and durability topics from gas diffusion layer materials and bipolar plate materials, a large part of the book will cover various aspects from membrane electrode assemblies, i.e., low and high temperature MEAs as well as DMFC MEAs. Since MEA Cost and Durability are heavily linked, this topic will be discussed one contribution. In the consecutive chapter on MEA and stack operation, the impact of contaminants (e.g. in the gas streams or water for humidification) on MEA and stack lifetime will be described. Furthermore, some information will be given on reliability and predictive testing of MEAs and stacks.