The level of adaptive diversity within an ecosystem or natural group is an aspect of biodiversity that is not necessarily available from counts of species or higher taxa. One method of estimating adaptive diversity is to measure the morphological disparity, or the range of variation observed in the sample. Morphological disparity can be used to investigate large-scale evolutionary changes in the fossil record, and has also been applied to a number of questions in extant organisms, ranging from investigations of diversification through time to understanding ecological community structure. In this dissertation, I investigated the potential for application of morphological disparity methods to understanding diversification in extant and fossil primates. I assessed the degree to which the fragmentary dental remains available for fossil mammals may represent skeletal diversity in other regions, and the extent to which ecological diversity in diet and activity pattern is captured by morphological disparity. I also investigated the relative influence of diet and size on dental morphology. Finally, I applied measurements of morphological disparity in the dentition to understanding the adaptive diversification of a radiation of primate-like mammals, the plesiadapiforms. Ecological distances and morphological distances were correlated in the two extant radiations examined, and gave similar assessments of the pattern of diversification through time in the extant radiations. Comparisons of morphological diversity in molar morphology compared to other regions indicated that, while not necessarily representative of diversity in other regions, molar morphological disparity is particularly useful for identifying adaptive diversity in both analyses of diversification through time, and identification of unusually diverse clades. Geometric morphometric analysis was found to be an effective method to assess dietary signal in molar morphology. Analyses of morphological disparity in plesiadapiforms indicate that over the course of the Paleocene sufficient diversity in molar morphology was accumulated such that the plesiadapiforms achieved levels of molar disparity comparable to those of extant groups such as strepsirrhines, even given their much shorter temporal range. Morphological disparity and taxonomic richness were not entirely concordant with maximum disparity in the Plesiadapoidea, with the highest taxonomic diversity occurring earlier than greatest morphological disparity. The latter result indicates that molar morphological variation may add information to analyses of diversity through time in this group, including evaluating hypotheses regarding extinction and diversification in this group and other fossil mammals.