Hexapoda (insects and their relatives) comprise over half of all described species, and demonstrates large variation in species richness among major sub-clades. This has led to various hypothesized controls responsible for structuring diversification within hexapod lineages, including morphological key innovations, dietary shifts (in particular plant feeding) and small body size. This thesis explores these ideas in the context of an explicit phylogenetic hypothesis for the group, constructed from published sequence data and literature derived constraints, and dated using a fossil calibrated relaxed molecular clock (Chapter 2). Based on this framework, models of the diversification process identify complete metamorphosis as a likely key innovation in the hexapod radiation, in addition to further up and down shifts in diversification rate responsible for the observed richness distribution (Chapter 3). Analysis also suggests that ideas regarding the role of plant feeding in diversification are related to restricted clade sampling, and a more comprehensive approach recovers no consistent association between particular diets and net diversification rates, in addition to heterogeneity in the age of dietary groups and in transition rates among dietary categories (Chapter 4). Our data also suggests body size evolution in hexapods occurs independently of clade richness, and is broadly dominated by neutral evolution on a log scale (Chapter 5). Thus, this thesis supports some hypotheses regarding controls on insect richness, whilst conflicting with other, well established ideas. It also provides a novel dated phylogenetic framework for further studies of hexapod evolution and identifies several novel directions for research into the origins and development of this diverse and important radiation.