In biology, few organs have been as elusive as the lung-air sac system of birds. Considerable progress has recently been made to fill the gaps in the knowledge. While summarizing and building on earlier observations and ideas, this book provides cutting-edge details on the development, structure, function, and the 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 uncertain issues. The specific goal here was to highlight the uniqueness of the design of the avian respiratory system and the factors that obligated it.