PI3K (phosphoinositide 3-kinase) regulates diverse cellular responses in the immune system, and members of this enzyme family are considered attractive drug targets for modulating allergy, inflammation and leukaemia. Clearly it is important to understand the function of PI3K in T-lymphocytes, cells that regulate nearly every aspect of immunity. However, the precise role of PI3K in T-cell development and function has been difficult to determine. In this review, I summarize current knowledge of PI3K function in T-cells, focusing on the class I subgroup of PI3K catalytic and regulatory isoforms. I discuss gene disruption studies in mice that reveal redundant or limited roles for individual isoforms, along with evidence for potential autoimmunity when class IA PI3K signalling is reduced.