Plants are equipped with a wide array of constitutive deterrents that include both toxic chemicals and mechanical barriers. In addition to these, there is a complex system of inducible defense responses aimed at stopping herbivores and pathogens. This chapter describes signals, their perception, and transduction in a typical plant defense response. The defense response consists of a large number of metabolic changes, including changes in the expression of many different genes—that is, a single elicitor signal sets into motion a complex cascade of events, possibly through an array of secondary signals. The extracellular signal compounds involved in triggering defense reactions are known as “elicitors.” The effect of an elicitor is to induce a whole array of responses in a parallel or sequential manner. Elicitors of plant defense responses fall into two categories depending on their source: exogenous and endogenous signals. Exogenous elicitors are the primary signals in plant pathogen interactions. Proteins, oligosaccharides, glycoproteins, fatty acids, and derivatives have been identified that can function as exogenous elicitors. Endogenous elicitors are components of the intercellular signal transduction system of plants. © 1994 Academic Press Inc.