This chapter describes the current understanding of the growth mechanism of plasma electrolytic oxidation (PEO) coatings on valve metals and non-valve metals (carbon steel, brass, and copper). An overview of the historical development of the PEO technique on valve and non-valve metals is first made. The second part discusses the growth mechanism of PEO on valve metals. Various influencing factors and the typical physical—chemical processes and characteristics of PEO coatings on different valve metals are introduced. PEO coatings on valve metals usually grow discretely under repeated discharges. However, soft sparking leads to uniform coating growth on Al. Two growth models are introduced. The third part introduces the PEO of non-valve metals, which is an area still in its infancy. Initially, PEO is thought to be unsuited for non-valve metals. However, recent studies show the successful PEO treatment on some non-valve metals. In these circumstances, stable plasma discharges are proved to be achieved through the deposition of an initial insulating layer of electrolyte species. The substrate participated less in the coating formation. It is still a long way to develop high-quality coatings on non-valve metals.