Mysticeti (baleen whales) is one of two major clades of Cetacea (whales and dolphins). Balaenopteridae (rorqual whales) is the most speciose and morphologically diverse group of crown mysticetes. Despite having the largest number of extant species of any mysticete lineage, as well as many fossil taxa, the phylogenetics of Balaenopteridae remain unresolved. This study investigates the evolutionary relationships of extant and extinct balaenopterids and places a new fossil species of Balaenoptera into a phylogenetic context. A new species of balaenopterid, Balaenoptera colcloughi sp. nov. is described from the upper Pliocene San Diego Formation (2-4 million years ago). Cranial and postcranial material from four specimens in the collections of the San Diego Natural History Museum is assigned to this species. The specimens of this new taxon represent an ontogenetic series (one adult and three sub-adults). The phylogenetic relationships recovered in this study are consistent with previous analyses. Monophyletic Balaenidae, Eschrichtiidae, and Balaenopteroidea were all recovered in the analyses. Balaenopteridae was found to be paraphyletic in most analyses conducted. The analyses also recovered previously recognized relationships among extant balaenopterids. A novel finding of these analyses was a close relationship between the extant Megaptera novaeangliae and Balaenoptera physalus, and extinct Balaenoptera colcloughi, Balaenoptera siberi and 'Megaptera' hubachi. The nesting of M. novaeangliae with B. physalus and other species of Balaenoptera is evidence that questions the recognition of a separate genus for the humpback whale, M. novaeangliae, from the rest of the balaenopterids (Balaenoptera). Divergence dating was used to investigate the temporal context of mysticete evolution. This study identified and utilized a phylogenetic and geologically supported set of fossil calibrations for Mysticeti by examining the impact of calibration choice. This set was tested against the commonly used fossils for calibration and it was found that calibration choice affects the level of confidence of age estimates. The use of external calibrations on dating mysticete divergence events was also found to improve the precision of the age estimates. The age estimates from these analyses were consistent with previous analyses. Balaenopteridae appears to have diverged as a clade in the middle to late Miocene, and this datum corresponds to a subsequent increase in mysticete diversity in the latest Miocene and into the Pliocene. While divergence estimates suggest a long history for some of the extant species, there is little to no fossil record of taxa assignable to extant Balaenoptera. Future work of improvement on divergence methods and descriptions of new fossil balaenopteroid taxa will be critical to improving the understanding of the evolutionary history of Balaenopteridae.