This document summarizes the characteristics of mammals, birds, and reptiles. It notes that mammals can be large or small, terrestrial or aquatic, are viviparous and feed on milk as infants. Birds have feathers, wings, lungs and are oviparous, though some like penguins cannot fly. Reptiles are cold-blooded vertebrates with scaly skin, lay eggs, and live in warm ecosystems to maintain their temperature. Examples of each group are provided.