A comprehensive experimental study aimed at characterization of the combined effects of high confinement and high strain rate on the deformation and strength of cementitious materials was conducted. Quasi-static triaxial compression tests for confining pressures ranging from 0 to 500 MPa were performed. Dynamic tests for strain rates in the range 60/s to 160/s under unconfined and confined conditions were conducted using a split Hopkinson pressure bar. A decrease in strain rate sensitivity with increasing confining pressure was observed. A new elastic/viscoplastic model that captures compressibility, dilatancy, and strain rate effects has been developed for concrete. There are no a priori limitations or restrictions regarding the specific expressions of the yield function and viscoplastic potential. A new flow rule was proposed. Procedures for determining the constitutive functions were developed. Comparisons between model predictions and data showed the proposed model describes with very good accuracy the high-pressure behavior of concrete.