The PI3K lipid kinases are involved in signal transduction and intracellular vesicular traffic, endowing these enzymes with multiple cellular functions and important roles in normal physiology and disease. In this mini-review, we aim to distil from the vast PI3K literature the key relevant concepts for successful targeting of this pathway in disease. Of the eight isoforms of PI3K, the class I PI3Ks have been implicated in the aetiology and maintenance of various diseases, most prominently cancer, overgrowth syndromes, thrombosis, inflammation and autoimmunity, with emerging potential roles in metabolic, cardiovascular and other disorders. The development of class I PI3K inhibitors, mainly for use in cancer and inflammatory disorders, is a very active area of drug development. In 2014, an inhibitor of the p110δ isoform of PI3K was approved for the treatment of some human B-cell malignancies. The key therapeutic indications of targeting each class I PI3K isoform are summarized and discussed.