The mechanism of avian respiration is still controversial. It is fundamentally different from respiration inmammals. Although during in- and expiration a continuous caudocranial airflow is present within the tertiary bronchiand the air capillaries of the avian lung, the air flow within the entire respiratory system is still equivocal. Severalpatterns explaining the air flow during in- and expiration have been proposed during the past century. Moreover,various anatomical structures and aerodynamic mechanisms have recently been described in an attempt to explainthe proposed mechanisms of respiration. This manuscript gives an overview of the anatomy of the avian respiratorysystem and the hypotheses concerning the physiology of avian respiration.