This chapter examines the presence, behavior, and separation of microplastics (MPs) in wastewater treatment plants, and how they remain partially removed despite extensive treatment. The chapter commences by exploring the occurrence of MPs in the influent, sludge, and effluent streams with a focus on varying concentrations of the different types of MPs (fibers and fragments), types of polymers (polyethylene, polypropylene, and polyethylene terephthalate), and population density and industrial discharges. It then explains what happens to the particles in the first, second, and third phases of treatment and how sedimentation, biological aggregation, and advanced filtration contribute to part of the removal. Retention or release depends on particle size, density, and shape; treatment processes may further fragment plastics, producing smaller and more mobile particles. The chapter also examines the partition between water and solids, where sludge serves as an important sink, particularly with large volumes and dense plastics. However, effluent still releases finer fractions into the water. Co-contaminant relationships, sludge management activities, and risks associated with land application are given special attention, as they may facilitate the movement of plastics to terrestrial habitats. The difficulty of quantification methodology and the necessity of unified standards have been mentioned, and the environmental impact conditions of MPs derived from wastewater treatment plants have been emphasized.