This chapter introduces fundamental microbiology from the perspective of environmental engineering, focusing on how microorganisms underpin biological wastewater treatment. It describes the structure and function of prokaryotic cells, the classification of bacteria, and their ecological roles in engineered systems. Key microbial groups—such as heterotrophs, autotrophs, nitrifiers, and denitrifiers—are presented in the context of their metabolic strategies and environmental requirements. The chapter also discusses microbial growth patterns, biofilm formation, and interactions with substrates, linking these concepts to practical applications in treatment processes. By simplifying complex microbiological principles into an engineering framework, it equips students with the foundational knowledge needed to understand and design bioprocesses for water quality management.