This book presents a comprehensive exploration of aquatic microbiology and biotechnology, emphasizing the relationships between microbial diversity, ecological processes, water quality, and environmental sustainability. It examines the roles of bacteria, archaea, viruses, algae, and protozoa in nutrient cycling, ecosystem productivity, and aquatic food webs. The text discusses microbial interactions, biofilms, aquatic pathogens, antimicrobial resistance, and microbial indicators used in water quality monitoring. Applied chapters address wastewater treatment, bioremediation, aquaculture health, and marine biotechnology as innovative approaches for environmental management. The book also highlights major global challenges, including climate change, harmful algal blooms, microplastics, and emerging contaminants, and explores the use of omics technologies, bioinformatics, and predictive modeling to support sustainable management of aquatic ecosystems and water resources.