The growing demand for clean water and rapid industrialization have highlighted the urgent need for effective wastewater treatment. Untreated or poorly managed wastewater poses significant threats to public health, aquatic ecosystems, and environmental sustainability. This chapter provides a comprehensive introduction to wastewater treatment, beginning with its importance in reducing pollution and conserving water resources. It defines key terminologies essential for understanding treatment processes, establishing a solid foundation for students, researchers, and professionals new to the field. The chapter also examines various wastewater sources, including domestic, industrial, and agricultural origins, and outlines their unique chemical and physical characteristics. Finally, the chapter offers an overview of both conventional and emerging treatment technologies, emphasizing the objectives of each stage and their roles in achieving safe and sustainable water reuse. This foundational chapter sets the stage for a deeper exploration of advanced treatment methods discussed in subsequent sections.