The name of inositol phosphatides or phosphoinositides is given to a rather heterogeneous group of substances having in common the presence of inositol, phosphoric acid, and fatty acid(s) among their constituents. The first evidence of their natural occurrence was obtained by Anderson, in 1930, who showed their presence in the lipides of the tubercle bacillus. Since then they have been found in many animal and vegetable tissues, and some of them have been isolated as relatively pure compounds. The majority, if not all of them, are acidic substances which, when prepared by the use of neutral solvents, are obtained as monophosphates of inorganic bases. The constituent inositol is present as a monoester of phosphoric acid, except in the case of brain diphosphoinositide, where it occurs as inositol metadiphosphate. With one possible exception, phosphoinositides contain 1 mole of glycerol per mole of inositol; less commonly, they contain carbohydrates and amines. While valuable information has been obtained on certain details of their chemical structure, in no case has the identification of a phosphoinositide been completed.