This project is research in mathematical physics. The essential thrust is the use of random walks to model quantum mechanical systems, with the aim of predicting what happens in large extended aggregations of particles on the basis of known short-range interactions. Specific topics to be investigated include Donsker-Varadhan theory, continuous symmetries, and quantum screening in Coulomb systems. The work on random walks might turn out to be important in a wide range of scientific and engineering problems that have no direct connection to mathematical physics.