Spondyloarthritis (SpA) encompasses a group of disorders linked by overlapping clinical manifestations and genetic predisposition. Newer classification systems developed for adults with SpA focus on identifying individuals with axial or predominantly peripheral involvement. All forms of SpA can begin during childhood, and can be considered on a continuum with adult disease. Nevertheless, there are important differences in presentation and outcome that depend on age at onset. This article highlights these differences, what has been learned about genetics and pathogenesis of SpA, and important unmet needs for future studies.