Paramagnetic metal ions offer outstanding opportunities for protein studies by nuclear magnetic resonance (NMR) spectroscopy. The paramagnetic effects manifested in the NMR spectra provide powerful restraints for the determination of the three-dimensional structure of proteins, open new possibilities for the analysis of protein-protein and protein-ligand interactions, and offer widely applicable tools for sensitivity enhancement of NMR experiments, resonance assignments, and studies of conformational heterogeneity and exchange. With the advent of new reagents for site-specific labeling of proteins with paramagnetic metal ions, a range of established and new strategies that used to be reserved for metalloproteins is becoming available for NMR spectroscopy of nonmetalloproteins.