Geologists and geophysicists have numerous ways to investigate and characterize heterogeneity in the earth. Geophysical characterization includes measurement of physical properties such as seismic velocities and density of rocks. Geological characterization includes mineralogical composition and grain size distribution that are both controlled by the processes by which the rock evolved. Geologists observe the surface of the earth and analyze rocks that originated from within the earth for signs of heterogeneity. The wide variation of rocks erupted from volcanoes provides geochemical and geological evidence of heterogeneity within the earth. Tectonic processes such as folding, faulting, and large scale crustal movements associated with plate tectonics contribute to making the lithosphere heterogeneous. Rocks recovered from boreholes show wide variation and rapid changes in chemical composition with depth. Geophysical measurements in wells show correlation and lack of correlation with chemical composition of the rocks, indicating that mineral composition alone is not the only factor that controls the physical properties of rocks. Deterministic seismic studies reveal a wide spatial variation in elastic properties within the earth’s lithosphere. Scattering of high-frequency seismic waves reveals the existence of small scale heterogeneities in the lithosphere.