This book examines a selection of philosophical issues in the context of specific episodes in the development of physical theories and presents scientific advances within their historical and philosophical contexts. Philosophical considerations have played an essential and ineliminable role in the actual practice of science. The book begins with some necessary introduction to the history of ancient and early modern science, but emphasizes the two great watersheds of twentieth-century physics: relativity and quantum mechanics. At times the term "construction" may seem more appropriate than "discovery" for the way theories have developed and, especially in later chapters, the discussion focuses on the influence of historical, philosophical and even social factors on the form and content of scientific theories.