Microplastic (MPs) pollution has become a crucial environmental issue, and wastewater systems are considered one of the primary sources of release and transportation of different plastic particles. This chapter discusses MPs’ sources, forms, and routes in domestic and industrial wastewater. Home operations, including laundry of synthetic fabrics, application of personal care products, house cleaning chemicals, and food packaging degradation, are significant sources of MPs. Additional MPs load contributors include industrial processes such as textile production, plastic handling, paints, finishes, and building construction. Other inputs include stormwater and urban run-offs, tire wear particles, road markings, litter, and atmospheric deposition. Another essential feature of the chapter is that the authors emphasize various types of MPs, such as fibers, fragments, beads, films, and foams, with references to their origin, properties, and ecological significance. Fibers prevail in the wastewater since they are generally found in large amounts in the textile industry, and foams have high ecological hazards mainly because of their floating nature, porosity, and ability to transport pollutants. Lastly, the differences worldwide, urbanization, and new sources of plastics are discussed to inform research, policy, and remediation approaches.