The phosphatidylinositol-3-kinase (PI3K) pathway has an essential role in signal transduction, where it is required for a number of different cellular processes including proliferation, differentiation, development, migration and growth. In the immune system, PI3K regulates inflammation by controlling the activation and recruitment of leukocytes. The generation of conditional knockout mice has allowed the study of PI3K isoforms specifically in B and T lymphocytes, and demonstrates the importance of intact signalling in their development and function. PI3K signalling must be tightly regulated in lymphocytes as excessive PI3K can lead to autoimmunity, immunodeficiency or cancer, whilst diminished signalling can result in developmental defects and immunodeficiency. Recent advances in the understanding of PI3K signalling have hastened the application of isoform-specific PI3K inhibitors, which are currently undergoing clinical trials. This review will focus on the p110δ catalytic subunit of the class 1A family of PI3K, and its role in the development and activation of B lymphocytes through various downstream effectors.