Quantum theory and relativity have been the most important development in physics during the twentieth century. Through a connected sequence of five essays, this book recalls some of the important personalities and events associated with the development and growth of quantum theory and relativity. Sketches of the lives of Paul Dirac, Niels Bohr and Eugene Wigner; features of the use of ideas of symmetry and invariance in contemporary physical theories, revolving around their descriptive, restrictive and creative roles, and mathematical aspects of quantum theory are presented. Some aspects of the interrelations between physics and biology are also discussed.