Phosphatidylinositol 3-phosphate (PtdIns(3)P), generated via the phosphorylation of phosphatidylinositol by phosphatidylinositol 3-kinase (PI 3-kinase), plays an essential role in intracellular membrane traffic. The underlying mechanism is still not understood in detail, but the recent identification of the FYVE finger as a protein domain that binds specifically to PtdIns(3)P provides a number of potential effectors for PtdIns(3)P. The FYVE finger (named after the first letter of the four proteins containing it; Fab1p, YOTB, Vac1p and EEA1) is a double-zinc binding domain that is conserved in more than 30 proteins from yeast to mammals. It is found in several proteins involved in intracellular traffic, and FYVE finger mutations that affect zinc binding are associated with the loss of function of several of these proteins. The interaction of FYVE fingers with PtdIns(3)P may serve three alternative functions: First, to recruit cytosolic FYVE finger proteins to PtdIns(3)P-containing membranes (in concert with accessory molecules); second, to enrich for membrane bound FYVE finger proteins into PtdIns(3)P containing microdomains within the membrane; and third, to modulate the activity of membrane bound FYVE finger proteins.