Respiratory organs have evolved to support gas exchange in a wide variety of environments. The diversity we see today is facilitated by the challenges of gas exchange in aquatic and terrestrial environments, in addition to metabolic requirements. When thinking about comparative lung morphology at environmental extremes, we need to acknowledge that the evolution of respiratory organs is due to environmental challenges. Evaginated respiratory organs dominate in the aquatic environment, with the transition to land facilitating the invagination of respiratory surfaces to protect against desiccation. Natural selection on the terrestrial lung to support endothermy led to increases in gas exchange surface area in mammals and birds, with birds evolving the most efficient respiratory system of all vertebrates to support flight. Adaptations in response to further environmental extremes (e.g., hypoxia, high-intensity exercise, diving, etc.) are overlaid on top of these evolved structures, with many similar responses occurring between respiratory structures (i.e. gills and mammalian lungs). This chapter highlights not only the evolution of different respiratory structures but also how environmental extremes challenge respiratory structure, function, and design.