Wave profile and equation of state (EOS) data are reported for low-porosity polycrystalline magnesium oxide under shock compression. The Hugoniot equation of state between 14 and 133 GPa is US =6.87(10)+1.24(4)up, where the numbers in parentheses are one standard deviation uncertainties in the last digit(s). Reverse-impact wave profiles constrain the compressional sound velocity, VP, at 10--27 GPa to ¿2%. Measured VP values are consistent with ultrasonic data extrapolated from 3 GPa. By combining the Hugoniot results with ultrasonic data, the adiabatic bulk modulus and its first and second pressure derivatives at constant entropy are 162.5(2) GPa, 4.09(9), and -0.019(4) GPa-1. The shear modulus and its first and second pressure derivatives are 130.8(2) GPa, 2.5(1), -0.026(45) GPa-1. Polycrystalline MgO has a compressive yield strength of 1--1.5 GPa at the elastic limit which increases to 2.7(8) GPa along the Hugoniot and is similar at unloading. Wave profiles for MgO at 10--39 GPa are described using a modified elastic-plastic model. There are significant differences in the dynamic response of single-crystal and polycrystalline MgO. ¿ American Geophysical Union 1995