The area of cell signaling has emerged as a major field of scientific focus, because of its central role in all aspects of normal cell function as well as in diseases and the effects of toxins and other pathogens in humans, animals, plants, and lower organisms. There are large numbers of proteins whose function is to mediate various aspects of cell signaling, including hundreds of cell surface receptors and an even wider array of transducer proteins and effector enzymes that generate second messengers or assemble multi-protein signaling complexes. These intermediate signaling molecules in turn regulate kinases and phosphatases, ion channels, transcription factors, and many other intracellular and cell surface targets. Together these pathways convert receptor activation into cellular responses, such as changes in gene expression, growth and differentiation, cell migration or contraction, secretion, and changes in membrane ion gradients and electrical properties. It is now recognized that these pathways are not linear but comprise a network of interconnected lines of communication that integrate inputs from a variety of extracellular and intracellular sources into an appropriate coordinated response. Alternatively, inappropriate activity of these receptors or signaling mediators can contribute to cell or organismal pathology or death. Finally, it is well-established that these receptors and signaling pathway molecules can be highly effective targets for drugs to prevent or treat diseases in humans and animals as well as for insecticides, herbicides, and other non-medical bioactive agents.