Sea anemones (order Actiniaria) are among the most diverse and successful members of the anthozoan subclass Hexacorallia, being found at all depths and latitudes and in all marine habitats. Members of this group exhibit the greatest variation in anatomy, biology, and life history in Hexacorallia, and lack any morphological synapomorphy. Nonetheless, previous molecular phylogenetic studies have found that Actiniaria is monophyletic with respect to other extant hexacorallians. However, relationships within Actiniaria have remained unresolved, as none of these earlier works have included sufficient taxon sampling to estimate relationships within Actiniaria. We have analyzed sequences from two mitochondrial and two nuclear markers for representatives of approximately half of the family-level diversity within the order, and present the first phylogenetic tree for Actiniaria. We concur with previous studies that have suggested that molecular evolution is unusually slow in this group. We determine that taxonomic groups based on the absence of features tend not to be recovered as monophyletic, but that at least some classical anatomical features define monophyletic groups.