Our understanding of the physical world was revolutionized in the twentieth century - the era of "modern physics." Two books by the second author entitled "Introduction to Modern Theoretical Foundations" and "Advanced Modern Theoretical Foundations," aimed at the very best students, present the foundations and frontiers of today's physics. Many problems are included in these texts. A previous book by the current authors provides solutions to the over 175 problems in the first volume. A third volume "Topics in Modern Theoretical Foundations" has recently appeared, which covers several subjects omitted in the essentially linear progression in the previous two. This book has three part 1 is on quantum mechanics, part 2 is on applications of quantum mechanics, and part 3 covers some selected topics in relativistic quantum field theory. Parts 1 and 2 follow naturally from the initial volume. The present book provides solutions to the over 135 problems in this third volume.The three volumes in this series, together with the solutions manuals, provide a clear, logical, self-contained, and comprehensive base from which students can learn modern physics. When finished, readers should have an elementary working knowledge in the principal areas of theoretical physics of the twentieth Introduction "Quantum " Solutions to the Schrodinger Equation Formal Developments "Applications of Quantum Mechanics 83: " Approximation Methods for Bound States Scattering Theory Time-Dependent Perturbation Theory Electromagnetic Radiation and Quantum Electrodynamics "Relativistic Quantum Field " Discrete Symmetries Heisenberg Picture Feynman Rules for QCD 231 "Problems and " The Two-Body Problem Charged Particle in External Electromagnetic FieldAdvanced undergraduate and graduate students in physics, science and engineering, researchers and engineers. Key The three volumes in this modern physics series provide a clear, logical, self-contained, and comprehensive base from which readers can obtain an elementary working knowledge in the principal areas of theoretical physics of the twentieth century The solutions manuals provide a teaching aid for instructors and a learning aid for students"