Abstract Near-infrared spectroscopy is usually considered to cover that part of the electromagnetic spectrum which falls between 0.71μ. and approximately 3 p. From the theoretical viewpoint, one might place the upper wavelength limit near 2.51μ. and define near-infrared spectroscopy as the study of low-energy electronic transitions and overtones and combinations of hydrogenic (C-H, N-H, 0-H, etc.) stretching and bending vibrations. This definition is favored by the fact that several commercial near-infrared instruments have an upper wavelength limit of about 2.65μ. Some instruments cover wavelengths up to 3.6μ) however, and the definition of near-infrared spectroscopy is sometimes broadened to include the study of fundamental hydrogenic stretching vibrations.