This chapter contains sections titled: General Properties of Intracellular Messenger Substances Cyclic AMP cGMP and Guanylyl Cyclases Metabolism of Inositol Phospholipids and Inositol Phosphates Storage and Release of Ca2+ Functions of Phosphoinositides Ca2+ as a Signal Molecule Diacylglycerol as a Signal Molecule Other Lipid Messengers: Ceramide, Sphingosine, and Lysophosphatidic Acid The NO Signaling Molecule