Segmental traits are not exclusive features of arthropods, annelids and vertebrates—the animals typically considered to be “segmented.” Other bilaterians show a great variety of repetitive structures arranged along the anteroposterior axis, such as folds in the body wall, transverse commissures in the nervous system, and evenly spaced muscle bundles. This chapter reviews the segmental traits present in each major bilaterian group describing, whenever possible, the cellular processes and molecular mechanisms involved in their formation during embryonic development.