Plasma electrolytic oxidation (PEO) is an efficient technique for producing a functional ceramic coating on the surface of metal matrix composites (MMCs), which can improve the corrosion resistance, mechanical and thermal protective properties of the material. This chapter focuses on the growth mechanism of PEO on aluminum (Al), magnesium (Mg), and titanium (Ti)-based MMCs under different reinforcement, electrolyte conditions, and power supply modes, respectively. By analyzing the current density–time (j–t) curve and spark discharge characteristics, the correlation between spark discharge intensity and coating growth is revealed. The chapter reviews that the type of enhancers, the composition of electrolytes, and the negative voltage significantly affect the characteristics of spark discharge and the microstructure of the coating. Such as, soft spark discharge under high negative voltage can effectively reduce defects and improve the density of the coating. In addition, the presence of reinforcement plays an important role in the growth kinetics of the coating. Thus, understanding these key factors will provide important references for developing high-performance MMCs and optimizing PEO processes.