The subject of the paper is to discuss and quantify deviations from reciprocity of the bidirectional reflectance distribution function (BRDF), i.e., the difference of BRDF obtained when inverting illumination and viewing directions. Directional reciprocity is not valid in general, because when the illumination beam has a spatial extension larger than the viewed area (as is most often the case for BRDF measurements), some of the scatterers building up the observed radiance are located at different places in reciprocal measurements. The physical systems under consideration in the two experiments are different, hence the breakdown of reciprocity. The paper develops a theory aiming at a quantitative estimation of deviations from directional reciprocity due to this factor. The theory is based on integral forms of the radiative transfer equation in a horizontal slab of heterogeneous absorbing and scattering media. The observed scene radiance is expanded in a series of scattering orders. Integral expressions of the single- and multiple-scattering radiance are derived and put in a form suitable for the analysis of the reciprocity problem. The first-order expression leads to an estimate of the order of magnitude ϵ of the relative deviations from reciprocity, ϵ≈h/D ΔQ/Q tan2 &thgr;i+tan2 &thgr;v-2 tan &thgr;i tan &thgr;v cos ϕ, where D is the size of the viewed area (pixel size for imaging sensors), h is the vertical photon mean free path, ΔQ/Q is a measure of the scene heterogeneity, and &thgr;i, &thgr;v, and ϕ are the illumination and view zenith angles and the relative azimuth between illumination and view directions. It is argued that this order of magnitude should remain approximately valid if all orders of scattering are taken into account. A discussion of practical applications in various fields, laboratory optical reflectometry, Earth radiation budget monitoring, and terrestrial surfaces remote sensing is given. ¿ 2001 American Geophysical Union