This chapter provides fundamental knowledge of water biology (microbiology) for the water practitioner. It describes and/or discusses different groups of microorganisms in relation to their size, shape, types, nutritional needs, and control. Most bacteria require organic food to survive and multiply. Waterborne bacteria are responsible for infectious epidemic diseases. Water treatment unit processes include: storage of raw water; prechlorination; coagulation—flocculation; water softening; filtration; and disinfection. Giardiasis ranks among the top 20 infectious diseases that cause the greatest morbidity in Africa, Asia, and Latin America. Although beavers are often blamed for contaminating water with Giardia cysts, that they are responsible for introducing the parasite into new areas seems unlikely. Patients with human immunodeficiency virus infection should be made more aware of the many ways that Cryptosporidium species are transmitted, and they should be given guidance on how to reduce their risk of exposure.