NMR is very well suited to the study of especially weak protein-protein interactions, as no crystallization is required. The available NMR methods to this end are reviewed and illustrated with applications from the recent biochemical literature: intermolecular NOEs, cross-saturation, chemical shift perturbation, dynamics and exchange perturbation, paramagnetic methods, and dipolar orientation. Most of these methods are now routinely applied for complexes with total molecular mass of 60 kDa and can likely be applied to systems up to 1000 kDa. A substantial fraction of complexes studied show distinct effects of induced fit affecting structural and dynamical properties beyond the contact interface.