Electrochemical energy conversion systems including polymer electrolyte membrane (PEM) fuel cells and electrolyzers are central to reducing global fossil fuel dependence through widespread implementation of renewable hydrogen in order to mitigate climate change. Great improvements have been made in the development of Platinum group metal (PGM) and PGM-free catalysts for PEM systems, however catalyst-ionomer interfaces that exist in these catalyst layers are not well understood. This workshop will provide forum for discussion of studies of model and real systems evaluating differences and similarities in the interfaces and catalyst-ionomer interactions across a variety of catalytic systems operating under acidic and alkaline conditions and elucidating the effects of catalyst and support chemistry and morphology on catalyst-ionomer interfaces and interactions. Of particular interest are investigations of the evolution of catalyst-ionomer interface under conditions relevant to device operation, development of best practices and protocols for catalyst-ionomer studies, improved electrochemical diagnostics, and novel physicochemical characterization approaches.