The central goal of our research is to advance the theoretical understanding of quantum mechanical processes in large molecules and the condensed phase. Unless severe approximations are introduced, direct solution of the Schrodinger equation is feasible only for small molecules, as it requires computational effort that increases exponentially with the number of particles. We develop new theoretical descriptions and simulation methods based on Feynman’s path integral formulation of quantum dynamics and apply them to investigate tunneling and coherence phenomena, proton and electron transfer reactions in solution and biological systems, the dynamics of quantum fluids, and excitation energy transfer in molecular aggregates.