Visible and near- and mid- infrared spectral measurements were made on intact and detached leaves of four crop species (Corn, Peanut, Soybean and Wheat) using two spectroradiometers under three different conditions : field measurements under artificial light and solar radiation conditions with and without an integrating sphere, and laboratory measurements with an integrating sphere. In the regions of 1350-1480, 1800-2000, and 2350-2500 nm wavelenghts, measurements of spectral reflectance (ρ) of crop leaves were not possible, even under fully sun-lit conditions due to atmospheric absorption of light in these regions. Relative water content (RWC, %) was well correlated with 1n (ρ1910), ρ1430, and 1n (ρ1650), while water content (WC, H₂O mg cm^-2) was correlated with ρ1200/ρ1430, ρ1650/ρ1430, ρ800-ρ1200 and ρ1100-ρ1200. Despite very high correlation between those indices and RWC for different crops, no common regression equation was obtained. On the other hand, a common linear relationship was obtained between spectral indices and WC for all four crops irrespective of leaf thickness, chlorophyll content and structure. The first derivative at 1121 nm was also found to be closely related to leaf water status. An obvious spectral shift of inflection point associated with water status was found aroud 2010 nm region. The location of inflection point shifted over a range from 2080 to 1880 nm according to the dehydration of leaves, and the degree of the shift was closely correlated with RWC and WC. Relative leaf water status of intact leaves of drought-stressed and well-watered soybean plants was estimated from spectral measurements in the field.