This book is devoted to the cosmological implications of the gauge theories of particle physics and of string theory. It presumes some prior knowledge of these subjects, such as that provided in the authors' previous books Introduction to Gauge Field Theory and Supersymmetric Gauge Field Theory and String Theory, but it is self-contained. Like the earlier books it is intended as an introductory textbook for a first course on the subject at graduate student level. Recent observations confirm that the universe is very flat and extremely homogeneous. The most plausible explanation of this is that the universe passed through an inflationary era. The constraints this imposes on the presumed underlying field theory are studied and the possibility of satisfying these in a supersymmetric theory or in supergravity theory is discussed. Finally, black hole solutions of the supergravity theory that approximates string theory at low energies are considered, and the insight that string theory affords into the microscopic origin of the Bekenstein-Hawking entropy is discussed. Cosmology in Gauge Field Theory and String Theory will provide a modern introduction to these important problems from a particle physicist's perspective.