Theoretical and computational approaches play a key role in developing and optimizing new materials and devices for energy storage and conversion applications. Here we use several examples to illustrate how theory and computations can help accelerate the design and development of materials for fuel cell and electrolyzers by addressing the issue of their durability. We emphasize computational studies of different processes that can lead to decreased durability of fuel cells and electrolyzers under operating conditions. These studies, for instance, include catalyst corrosion [1], the change in the catalyst structure at different cell potentials and pH [2], poisoning of the catalyst with a crossover fuel or fragments from the ionomeric binder [3,4], and phosphoric acid leaching in acid-doped high temperature fuel cell polymer electrolytes [5].We first illustrate how combination of experimental and first principles thermochemical data can be used to predict and understand the stability of catalytic materials in aqueous media as a function of pH, cell potential, and temperature. Examples will include transition metal carbides and nitrides [1] and the size effect on the stability diagrams of precious metal catalysts [2]. Secondly, we discuss how first principles calculations can be used in the design of high temperature membrane fuel cells. The specific example will include a study of cluster energetics between phosphoric acid, water, and proton-accepting or hydroxide-donating bases, which are explored in the design of acid-doped polymer electrolytes [5].[1] I. Matanovic, F. H. Garzon, N. J. Henson, J. Phys. Chem. C, 115, 10640–10650 (2011).[2] I. Matanovic, F. H. Garzon, J. Electrochem. Soc. 167, 046518 (2020).[3] D. Sebastián, A. Serov, I. Matanovic, K. Artyushkova, P. Atanassov, A.S. Aricò, V. Baglio, Nano Energy, 34, 195-204 (2017).[4] I. Matanovic, S. Maurya, E. J. Park, J. Y. Jeon, C. Bae, Y. S. Kim, Chem. Mater, 31, 11, 4195-4204 (2019). [5] I. Matanovic, A. S. Lee, Y.-S. Kim, J. Phys. Chem. B, 124, 7725-7734 (2020).