In the 20th century — the age of "modern physics" — our view of the physical universe underwent a revolution. This book, Developments in Modern Physics, presents the fundamentals and frontiers of contemporary physics to the most gifted pupils. Typically, students must take multiple courses to cover the majority of these topics. The purpose is to give them a sense of where they're heading and how everything fits together as they go. Quantum mechanics. nuclear physics, particle physics, special relativity, condensed-matter physics, relativistic quantum mechanics, and general relativity are the main topics covered in this book. The objective is to address these topics in considerable depth such that students "get" them and have a basic understanding of them. The book presupposes a one-year calculus-based introductory physics course and a one-year calculus course. Several appendices educate the reader on any additional mathematics that may be required. Numerous problems are presented, the majority of which will lead dedicated readers as far as they wish in modern physics.