This project entails the development and use of methods to calculate the properties of neutron matter, nuclear matter, and atomic nuclei, with applications to both nuclear structure and to astrophysical systems such as neutron stars. The Schroedinger equation is solved in imaginary time using Monte Carlo sampling techniques to deal with the large dimensionality of these quantum mechanical systems. Both space and spin coordinates are sampled to allow the method to be applied to large systems of order 100 particles. Accurate two-body potentials and empirical three-body potentials are used to describe the nuclear forces. Important aspects of this work include the development of techniques to study the pairing transition that leads to superfluidity in neutron matter; the study of the structure of large nuclei; and eventually to be able to treat hot nuclear matter. The project also will support and train a graduate student in nuclear many-body theory.