Nitric oxide (NO) is the simplest, lightest and only gaseous signal transmitting molecule in animals. In recent years, there have been several comprehensive studies examining the role of NO in plant growth, metabolism and defense responses, This compilation, following a brief history of the emergence of NO research, will chiefly summarize what is currently known about NO signaling pathways and describe the potential sources of endogenous NO, its biosynthesis, metabolism and transport, particularly in plants. This work also examines the role of NO in plant growth and development, its interaction with hormones, and its activity in response to abiotic stress conditions. Many studies employing pharmacologic and biochemical methods have shown that NO is functional in programmed cell death, stoma closure, and seed germination. These functions are particularly defense-related gene expression in plants. That the role of NO in plant biology was proven by newly developed methods adds an ever growing importance to this messenger molecule.