Symmetry considerations dominate modern fundamental physics, both in quantum theory and in relativity. This book presents a collection of philosophy-on-physics papers, first compiled in 2003, highlighting the main issues and controversies, and providing an entry into the subject for both physicists and philosophers. It covers topical issues such as the significance of gauge symmetry, particle identity in quantum theory, how to make sense of parity violation, the role of symmetry-breaking, the empirical status of symmetry principles, and so forth, along with more traditional problems in the philosophy of science. These include the status of the laws of nature, the relationships between mathematics, physical theory, and the world, and the extent to which mathematics dictates physics. A valuable reference for students and researchers, it will also be of interest to those studying the foundations of physics, philosophy of physics and philosophy of science.