This book presents a selection of papers on nuclear magnetic resonance, electron paramagnetic and ferromagnetic resonance written mostly during the first two decades (1945-1965) of the development of these scientific fields of endeavour. The focus of the papers is on magnetic relaxation phenomena, spin interaction in condensed matter, and on solid state masters. The collection started with reproduction of the PhD thesis Nuclear Magnetic Relaxation , Leiden, 1948. Brief comments by the author put each paper in an historical perspective.