Starting from a comprehensive quantum mechanical description, this book introduces the optical (IR, Raman, UV/Vis, CD, fluorescence and laser spectroscopy) and magnetic resonance (1D and 2D-NMR, ESR) techniques. The book offers a timely review of the increasing interest in using spin-label ESR as an alternative structural technique for NMR or X-ray diffraction. Future aspects are treated as well, but only as an illustration of the progress of ESR in this field.