The aim of this Special Issue is to provide a series of original research and review papers focused on Plasma Electrolytic Oxidation (PEO) Coatings applied to aluminium, magnesium, titanium, steel and zinc substrates for a wide array of applications. Contributions from leading researchers worldwide cover the following key topics: A fundamental understanding of the PEO process: a mechanistic study of and modelling of coating growth; The properties and performance of PEO coatings: corrosion, wear, luminescence; Hybrid PEO coatings; Functionalization of PEO coatings; Active protection based on PEO; Bio-applications of PEO coatings; Advanced PEO processes.