The isolation of a “diphosphoinositide” fraction from ox brain by Folch (1949) and the observation by Dawson (1954) that radioactive phosphate was rapidly incorporated into its lipids laid the foundations of our present knowledge of the brain phosphoinositides. The structures of the major phosphoinositides, phosphatidylinositol (I), phosphatidylinositol-4 phosphate (II) and phosphatidylinositol-4,5-biphosphate (III) are given in Figure 1. The generally accepted abbreviations used in this chapter are Ptdlns (I), PtdIns 4-P (II), and PtdIns 4,5-P2 (III).