The thecostracans include the Facetotecta, Ascothoracida, and Cirripedia and show great diversity in both morphology and biology. This makes them ideal models for studying evolutionary adaptations of the larval and adult body-plan, lifestyle, and reproduction. Surprisingly, despite all the work published since Darwin's seminal monographs, few studies have tested evolutionary hypotheses about Thecostraca within a phylogenetic context. In this review, we combine a Bayesian phylogenetic method and multilocus sequence data to reconstruct the evolutionary history of 12 key thecostracan phenotypic traits associated with their lifecycle, larval biology, reproduction, and adult morphology. Our analyses show that thecostracan biological diversity resulted both from unique innovations and from events of convergence. This provides an opportunity to reevaluate previous classifications of the Thecostraca and the theories relating to the origin and diversification of this taxon.