Signal transduction is any process by which a cell converts one kind of signal or stimulus into another. Processes referred to as signal transduction often involve a sequence of biochemical reactions inside the cell, which are carried out by enzymes and linked through second messengers. In many transduction processes, an increasing number of enzymes and other molecules become engaged in the events that proceed from the initial stimulus. Responses of cells to environmental signals, toxins and stressors have profound implications for diverse aspects of human health and disease including development, cystic fibrosis, diabetes, asthma, heart, autoimmune diseases and cancer. The delineation of the signal transduction pathways affected in these and other complex human diseases are likely to present new avenues for therapeutic intervention and understanding of human disease mechanisms.