Low-molecular-weight chemicals produced and secreted by neurons and a limited range of non-neural cells are the major vehicles of communication between nerve cells and other neurally controlled non-neural cells. Research over the past several decades has identified a wide spectrum of such \"neuromediators\", delineated their structures, elucidated pathways of biosynthesis and biodegradation, and determined the processes of development and function that they influence physiologically. These studies have revealed the nature of cellular receptors that recognize neuromediators selectively and the pathways that transduce signals from neuromediator-occupied receptors to the target cells. For most neuromediators and their receptors, pharmacological agonists and antagonists of high specificity also have been discovered or designed and synthesized. This volume examines the conceptual, methodological, and practical progress in the field from broader biological and pathophysiological perspectives.