The red edge is the sharp change in leaf reflectance between 680 and 750 nm and has been measured on leaves of a variety of species by first derivative reflectance spectrophotometry. A parameter λrewas defined as the wavelength of maximum slope and found to be dependent on chlorophyll concentration (p<0.001), with additional effects of species, developmental stage, leaf layering and leaf water content. The maximum slope parameter was found to be independent of simulated ground area coverage. Often two λrecomponents were identified and both showed a linear relationship with the chlorophyll content of the leaves. The results have been interpreted in terms of Beer's Law and Kubelka-Munk theory. The chlorophyll concentration dependence of λreseems to be explained in terms of a pure absorption effect, and it is suggested that the existence of two λrecomponents arises from the additional factor of leaf scattering properties. In comparison to broad band reflectance, the results indicate that red edge measurements will be valuable for assessment of vegetative chlorophyll status and leaf area index independently of ground cover variations, and are particularly suitable for early stress detection.