Discontinuous molecular dynamics simulation is used to study the phase behavior of diblock copolymer/nanoparticle composites. The copolymers are modeled as chains of tangent hard-spheres with square shoulder repulsions between unlike species, while the nanoparticles are modeled as hard-spheres with a square shoulder repulsion with one of the copolymer blocks. The resulting phase diagrams are presented for composites containing nanoparticles of various sizes and interaction strengths and include lamellae, perforated lamellae, cylinders, and disordered phases. Composites containing large nanoparticles also exhibit two-phase coexistence between different copolymer phases or between a copolymer phase and a nanoparticle phase, depending on the nanoparticle interaction strength. We also present concentration profiles perpendicular to the lamellar interface for nanoparticles of different sizes and interaction strengths. Neutral nanoparticles concentrate at the interface between copolymer domains, while interacting nanoparticles concentrate within the favorable domain. The larger nanoparticles are more easily localized but have less impact on the copolymer concentration profiles. The lamellar spacing increases with nanoparticle volume fraction for interacting nanoparticles but decreases with nanoparticle size. The locations of the phase transitions are in qualitative agreement with theoretical predictions, but the concentration profiles are inconsistent with theoretical predictions. The variation of the spacing with nanoparticle volume fraction is consistent with experimental data.