Hormones, neurotransmitters, cytokines and a number of extracellular ligands participate in extracellular signaling mechanisms that modify cellular biochemistry. Intracellular ligands are involved in an array of intracellular signaling processes that increase or decrease the effects of extracellular ligands. Both extracellular and intracellular mechanisms are important in drug effects, maintaining health and treating chronic disease. The management of chronic diseases is becoming a very common process for many people. Many questions remain to be answered about chronic diseases. What causes them? Is it possible to prevent them? How are they best treated?