The signal from a confocal measurement as the focal volume is scanned down into a tissue yields an exponential decay versus depth z focus, signal = ρ exp(-μ z focus), where p [dimensionless] is the local reflectivity and μ [1/cm] is an attenuation coefficient. A simple theory for how ρ and |a depend on the optical properties of scattering (μ s) and anisotropy (g) is presented. Experimental measurements on 5 tissue types from mice (white and gray matter of brain, skin, liver, muscle) as well as 0.1-μm-dia. polystyrene microspheres are presented. The tissues have similar μ s values (about 500 [1/cm]) but variable g values (0.8-0.99). Anisotropy appears to be the primary mechanism of contrast for confocal measurements such as reflectance-mode confocal laser scanning microscopy (rCLSM) and optical coherence tomography (OCT). While fluorescence imaging depends on fluorophores, and absorption imaging depends on chromophores, the results of this study suggest that contrast of confocal imaging of biological tissues depends primarily on anisotropy.