The disciplines of particle physics, cosmology and mathematics are coming together today in remarkable ways. Research in theoretical elementary particle physics and string theory will examine how the world works, from the strings that may lie deep inside the smallest particles to the edge of the visible universe and beyond. The tools developed in doing this may influence the nature of mathematics as well. The areas to be studied include the large-scale structure of the universe, the nature of space- time, the fundamental forces of nature and the most basic constituents of matter. Both phenomenologically and with powerful new techniques arising from string theory, the research will study the properties of matter, such as revealed at the largest accelerators (the Tevatron, and in the next decade, the Large Hadron Collider). This endeavor should explain the origin of particle masses. In addition, the work will seek the underlying explanation of CP violation -- the difference between the behavior of matter and that of antimatter. This is a tiny effect soon to be explored further at the BABAR collider in Stanford, but it is likely to explain why we are made of matter, not antimatter. Other challenges to be addressed involve the physics of supersymmetry (a striking, but still hypothetical prediction of string theory), the quantum mechanics of black holes, and the physics and mathematics underlying string theory. These researches will deepen our understanding of the fundamental processes occurring in the universe and lead to new insights in mathematics and quantum field theory.