Abstract Use of Landsat vegetation indices (combinations of spectral bands) to measure standing crop biomass production in rangelands is addressed. The theoretical basis for vegetation indices is briefly reviewed for ratios, differences and normalizations. Both optical and digital indices are discussed. The reliability of indices as indicators of rangeland primary production is evaluated in relation to drought impact. Seasonal green-up and senescence cycles are shown to be important factors when using indices as stress or drought indicators. Experimental results for the short grass prairies of Colorado show the onset and recovery from drought from 1973 to 1978. The use of vegetation indices to monitor the changing quantity and condition of the vegetative cover of the earth's surface is one of the most promising applications of Landsat data. This application appears to be reliable and inexpensive and uses the attributes of multispectral information, rather than suffering from the loss of spatial information. By responding to the primary productive functioning of plants, the indices provide a measure that is difficult to make in any other way. Rather than being a substitute for more conventional measurements, the vegetation index provides a unique measure of vegetation that will be of value to many users for many applications, including the monitoring of drought impact.