In biology, few organs have so eluded understanding as the lung-air sac system of birds. Considerable progress recently has been made in filling the gaps in knowledge, however. While summarizing and building on earlier observations and ideas, this book provides cutting-edge details on the development, structure, function, and evolutionary design of the avian respiratory system. Outlining the mechanisms and principles through which biological complexity and functional novelty have been crafted in a unique gas exchanger, this account will provoke further inquiries on the many still-unanswered questions. The specific goal of the book is to highlight the uniqueness of the design of the avian respiratory system and the factors that led to its development.