Summary: Bees (Hymenoptera : Apoidea : Anthophila) are a speciose and functionally diverse clade of insects, comprising over 20,000 described species which act as key pollinators across terrestrial and agricultural ecosystems. However, despite their ecological and economic importance, we still lack fundamental understanding of the consequences of bee functional and life history variation for bee ecology, evolution, and pollination services. As native pollinator abundance and diversity continue to decline worldwide, expanding our understanding of how trait variation among bees shapes their geographic distributions and dietary preferences will enhance our ability to predict how global changes may impact this crucial ecosystem service. In this dissertation, I seek to address these gaps by adopting a macroecological and macroevolutionary approach to bee trait diversity. I will integrate large datasets of traits and phylogenies to identify the environmental factors that govern variation in key functional traits and explore how this variation influences bees' interactions with flowering plants. I will also generate new data on diet preferences for poorly studied tropical bees using novel approaches for honey and pollen metabarcoding and contribute to ongoing efforts in the field to create large, global datasets of bee traits and bee-plant interactions. Ultimately, this research will provide new insights into the ecological processes shaping bee diversity and their interactions with flowering plants.