This chapter commences with briefly addressing the various types of solid polymer electrolytes (SPEs), including ionically conducting phases, organically modified ceramic polymers, polymers in which nitrogen acts as a mediator for proton conduction, sulphonated polymers, and some anion-exchange materials. Emphasis is on sulphonic-containing perfluorinated ionomers, such as Nafion. A brief overview is then given of the many diverse fields of application of micro-heterogeneous SPE-based membranes in the processes involved in eletrocatalysis and water electrolysis. The chapter concludes with mention of current trends in research and possible future applications of electrocatalytic membrane processes.