Plastic particles less than 5 mm in diameter are called Microplastics (MPs), which have become a common pollutant of international interest because of their durability, mobility, and ecological hazards. They can be widely generalized into primary MPs, which are purposefully prepared to be microscopic (e.g., microbeads, pellets), and secondary MPs, which are made during the breakdown of larger plastic waste. Their worldwide distributions are now well known in freshwater, marine, atmospheric, and terrestrial environments, all noted as essential reservoirs. Wastewater, specifically, is one of the most important routes, both as a sink and a microplastic carrier of MPs. Wastewater systems combine the contributions of domestic processes, industries, stormwater, and atmospheric deposition, and thus are the focal points of the multiplicity of microplastic sources. Even though MPs are partly trapped at wastewater treatment plants, their low size, buoyancy, and chemical resistance allow significant proportions to bypass treatment and re-enter water and soil environments. Emitting treated effluents into rivers and lakes and applying sewage sludge onto the land promote the reallocation of MPs among the ecosystems. The study of their behaviour in wastewater helps to define the most critical environmental routes and emphasize the importance of the measures on human health conditions, the balance of the ecological system, and the necessity of successfully monitoring wastewater. This chapter introduces the concept of MPs and their relevance to sewage. Additionally, it also gives a spotlight on the Sustainable Development Goals related to the MPs.