The inositol lipids PI(4,5)P-2 and PI(3,4,5)P-3 are important regulators of actin polymerization, but their different temporal and spatial dynamics suggest that they perform separate roles. PI(3,4,5)P-3 seems to act as an instructive second messenger, inducing local actin polymerization. PI(4,5)P-2 appears to be present at too high a concentration and homogenous a distribution to fulfil a similar role. Instead, we suggest that PI(4,5)P-2 acts permissively, restricting new actin polymerization to the region of the plasma membrane.