Early 19th century classifications of the predaceous diving beetle subfamily Agabinae have been continuously reworked by subsequent systematists, but remain in flux at all taxonomic levels. Morphological characters have not produced sufficient evidence for a robust phylogeny, as there are few informative characters on which to base the taxonomy below the level of subfamily. It has been 12 years since the only comprehensive analysis of the molecular systematics and biogeography of the subfamily was published by Ribera et al. (2004). In the current project I update the molecular systematics of the Agabinae, addressing problems of biogeography, diversification rate, and taxonomy, at the subfamily, genus, and species levels. I apply dense taxon sampling, including data from the previously underrepresented Neotropical, Northern Nearctic, Eastern Palearctic, and Afrotropical regions, to a dataset of 10 gene sequences; (five mitochondrial, five nuclear). Using Bayesian and likelihood approaches, as well as fossil data, I infer time-calibrated phylogenies for the subfamily and for the genus Agabus. My results indicate the Agabinae originated during the early Paleogene in the Northern Hemisphere, with expansions to Australasia and South America likely via long distance dispersal. The tribes Agabini and Platynectini were consistently recovered as monophyletic, as was Hydrotrupini; however, results suggest that Hydrotrupes should be elevated to subfamily Hydrotrupinae. Relationships within and between genera were consistently recovered with strong Bayesian support with the exceptions of the relationship between Platambus and Agabus, species groups of Ilybius and Agabus, and for the subgenera of Agabus. In addition to polyphyletic Platambus occurring in Agabus, 44% of the current Agabus species groups (not monotypic) were not recovered as monophyletic. Collections in Northern Canada and Alaska yielded new locality data for 9 species, and identified possible refugial populations of Agabus arcticus (Paykull, 1798) in the Peel watershed (Yukon Territory). Further study of A. arcticus in the Nearctic will likely support the recognition of unique Nearctic subspecies. Molecular phylogenetic results provide new insights into Agabinae systematics, and also find support for many taxonomic hypotheses developed by the pre-cladistic workers in the group, underscoring the importance of integrative taxonomy combining molecular (this project) and morphological data (past workers).