Reflecting the oeuvre of "a man of two cultures: the culture of pure mathematics and the culture of theoretical physics" (in the words of his long time friend and co-author, Kameshwar Wali), this volume is centred around the notion of symmetry and its breaking. Starting with particle physics, the contents proceeds to symmetries of matter, defects, and crystals. The mathematics of group extensions, non-linear group action, critical orbits and phase transitions is developed along the way. The symmetry principles and general mathematical tools provide unity in the treatment of different topics. The selected papers and lecture notes are preceded by a lively biography of Louis Michel and a commentary that relates his selected works with the contemporary trends of his time. This book should be of interest to theoretical physicists, chemists, applied mathematicians and historians of science, and is accessible to graduate (and advanced undergraduate) students.