Metabolism of phosphoinositides by phosphoinositide-metabolizing enzymes. Shown is the relationship between different phosphoinositides and their metabolizing lipid kinases (red arrows), lipid phosphatases (blue arrows), and PLC (green arrows). In this review, we focus on the signal transduction mediated by the lipid kinases PIP5KI and PI3K, and the PLC in platelets. PIP5KI, phosphatidylinositol-4-phosphate-5-kinase type I; PI3K, phosphatidylinositol-3-kinases; PLC, phospholipase C.