Aquatic insects are the dominant invertebrate fauna in most freshwater ecosystems, and these insects figure prominently in the work of researchers, students, and managers, from diverse backgrounds. This broad-based reference text brings together a huge and scattered literature that presents recent advancements in the basic biology of aquatic insects from around the world — information that is not available in standard texts on entomology, freshwater ecology, or ecohydrology. Because aquatic insects have complex life cycles, they must master both terrestrial and aquatic environments, and also cross these ecosystem boundaries during different stages of development and reproduction. Consequently, this text presents the essential, relevant basics of entomology, but also topics unique to aquatic species. Life in and on top of the water surface are covered in unusual detail, including the biomechanics of life in water, locomotion on surface films and under water, gas exchange, physico-chemical stressors, feeding underwater, sensory perception and communication, reproduction, egg-laying and development, and the evolution of aquatic habits.