Reptiles were the first vertebrates to be adequately adapted for air-breathing and terrestrial habitation. Reptiles had developed an amniotic egg and through heavy keratinization of the epidermis a waterproof external cover. With the “sealing-off” of the epidermis, gas exchange was confined to the lungs. The modern lung with a dual role of oxygen and carbon dioxide exchange as well as acidbase regulation is first encountered in reptiles. Fundamental differences occur between the structure and function of the respiratory systems of modern reptiles and those of birds and mammals. Both the bronchoalveolar lung of mammals and the parabronchial one of birds are envisaged to have evolved from the highly complex multicameral lung of reptiles. As clearly evident from reptilian ascendancy over amphibians, evolution from one major animal group to another entails comprehensive changes of traits that range from structural, physiological, to behavioral ones.