This chapter focuses on the role of the gill in gas exchange, reviewing aspects related to ventilation, morphology, contact with the external environment, diffusion across membranes, blood flow and perfusion, and diffusion at the tissue level. The external environment and lifestyle of the fish will dictate efficiencies or inefficiencies in gas exchange, which can be elaborated upon by discussing each step of the O₂ or respiratory gas transport cascade. From a physical perspective, gill filaments and lamellae can act like a sieve, being numerous or even fused at the tips to maximize gas exchange with the water. Gas exchange at the tissues is determined by perfusion of the tissues with blood and diffusion across capillary membranes to and from the metabolizing tissues. Representing over half of all extant vertebrates and over 400 million years of evolutionary history, the fishes, indeed, provide a fascinating group of organisms in which to investigate the array of adaptations at the level of gas exchange.