The Bilateria are a major group of animals, including the majority of phyla; the most notable exceptions are the sponges and cnidarians. For the most part, Bilateria have bodies that develop from three different germ layers, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderms, which are radially symmetrical as adults, but are bilaterally symmetrical as larvae.

Except for a few primitive forms, the Bilateria have complete digestive tracts with separate mouth and anus. Most Bilateria also have an internal body cavity, called a coelom when it lies within the mesoderm, and a pseudocoelom otherwise. It was previously thought that acoelomates gave rise to the other group, but it now appears that in the main acoelomate phyla (flatworms and gastrotrichs) the absence is secondary.

There are two main lineages of Bilateria. The deuterostomes include the echinoderms, hemichordates, chordates, and possibly a few smaller phyla. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are a number of differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes.