This chapter discusses the general characteristics of marine and freshwater environments and some of the microorganisms that live there. Life in aquatic environments is most impacted by the levels of oxygen and nutrients present. These are determined by physical properties and by the activities of microorganisms. Water is an excellent vehicle for the transmission of diseases, and the chapter also explores the measures taken to ensure the availability of safe drinking water. The chapter concludes with a discussion of the contamination of groundwaters by domestic wastes and the use of home wastewater treatment systems.