We present a study of inter- and intra-familial relationships of the carcharhiniform shark family Triakidae aimed at testing existing hypotheses of relationships for this group and at improving understanding of the evolution of reproductive traits in elasmobranchs. Our analyses and conclusions are based on evidence from DNA sequences of four protein-coding genes (three from the mitochondrial genome and a single copy nuclear gene) from eight of the nine genera and 20 of the 39 species currently assigned to the Triakidae. The sequence data offer strong support for the following previously proposed triakid clades: Galeorhinini (Hypogaleus+Galeorhinus); a subset of the Iagini (Furgaleus+Hemitriakis but not Iago); and part of the Triakinae (Mustelus, Scylliogaleus and part of Triakis). Interestingly, the molecular data provide considerable evidence of paraphyly of the genera Triakis and Mustelus. Our results suggest that the subgenera Triakis and Cazon of Triakis represent two distinct lineages that are only distantly related and that the genus Mustelus as currently defined does not constitute a monophyletic assemblage unless S. quecketti and some species of Triakis (subgenus Cazon) are included in Mustelus. Within our sample of species of Mustelus (including Cazon and Scylliogaleus), the sequence data support two well-defined clades that can be diagnosed by mode of reproduction (placental vs. aplacental species). The phylogenetic framework presented here is used to infer key events in the evolution and loss of placental viviparity among carcharhiniform sharks.