Bidirectional Reflection Distribution Function (BRDF) facilities at ERIM have been employed since the late 1960s to characterize the spatial, polarization, and spectral content of the scatter from target/background materials. This paper explores the "lessons learned" over the years, concentrating on the characterization of calibration standards, the expected variation of BRDF of materials, and the utility of BRDF data in the prediction of material/complex object radiative transfer. Expected and unusual material characteristics as a function of wavelength and polarization will be reviewed, as well as the procedures that have been used to provide accurate and repeatable BRDF data. The laboratory equipment that has been used will be briefly discussed, emphasizing the necessary characteristics and functions of the hardware needed to gather valid BRDF data. Finally, the utility/assessment/requirement of BRDF data to support modeling activities will be presented from the "ERIM" image/sensor information exploitation standpoint.