Phosphoinositide 3-kinases catalyze the addition of phosphate at the 3-OH position of inositol lipids in cellular membranes. Initially identified as an enzymatic activity associated with transforming oncogenic proteins, PI3-kinases are now known to be involved in signal transduction through receptors with intrinsic or associated tyrosine kinase activity (such as sre type tyrosine kinases) and receptors linked to heterotrimeric G proteins. PI3-kinases also have a role in vesicular trafficking events. PI3-kinases form a large family of related proteins, conserved in organisms as distant as yeast and man. The exact role of their lipid products remains to be established, but the multiplicity of cellular events in which PI3-kinases are implicated, together with their conservation in evolution, indicates that they fulfil an essential role in cellular physiology.