Seismic observations of the amplitude ratio amplitude P'670P'/Amplitude, P'P' suggest a reflection coefficient R~7-13% for reflections from the underside of the 670-km discontinuity. This can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region. Velocity and density profiles through the transition region were calculated for olivine, pyroxene, garnet, and olivine+garnet model mineral assemblages. For models with identical upper and lower mantle compositions, the computed reflection coefficients from the phase transitions pertinent to the 670-km discontinuity are less than 0.5%. These phase transitions produce their maximum R if they are effectively discontinuous, which gives R~2-3%, except for the garnet to perovskite transformation which gives R~8%. In general, given realistic impedance contrasts across a phase transition, the change in material properties must occur over a region less than 3 km thick for R to be of the order of 5%. A discontinuous change in chemical composition at 670-km depth, coupled with a change in phase, can also produce a reflection coefficient of the observed magnitude: R~2-3% for a first-order change in phase at 670 km with different iron concentraitons on each side of the discontinuity, and R~4-6% for cases with a discontinuous increase in silica content. Based on our analysis, the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient. Thus although limited by considerable uncertainties, the mineralogical and seismic data suggest a two-layered model of the mantle with a discontinuous change in both phase and composition at 670 km.