Bumble bees are an outstanding model system in which to study the organization and evolution of complex social behaviour. Bumble bees pass through several distinct phases during their annual life cycle, including solitary and eusocial phases, and the final stage of the colony cycle is marked by intense competition among the queen and workers over the production of males. Furthermore, there are approximately 250 species of bumble bees, and of the few species that have been examined, it is clear that multiple life history strategies are possible. Thus, the ultimate and proximate mechanisms underlying co-operation, conflict, and behavioural plasticity can readily be examined in bumble bees. Here, we describe the current state of knowledge about the evolutionary, ecological, behavioural, physiological, chemical, and genomic mechanisms and factors underpinning bumble bee social behaviour throughout the colony cycle. We highlight long-standing questions in the field and discuss how advances in genomics and comparative approaches across bumble bee species can provide profound insights into this fascinating system and the evolution of social behaviour.