Avialae is usually defined as all theropod dinosaurs more closely related to modern birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used.

Archaeopteryx lithographica, from the late Jurassic Period Solnhofen Formation of Germany, is the earliest known avialan which may have had the capability of powered flight.However, several older avialans are known from the late Jurassic Tiaojishan Formation of China, dated to about 160 million years ago.