NMR spectroscopy and X-ray cristallography are currently the only techniques capable of determing the structures of biological macromolecules like proteins and nucleic acids at atomic resolution. In addition, it is possile to study time dependent phenomena with NMR, such as intramolecular dynamics in macromolecules, reaction kinetics, molecular recognition or protein folding.
The basic phenomenon of NMR was discovered in 1945: The energy levels of atomic nuclei are split up by a magnetic field. Transitions between these energy levels can be induced by exciting the sample with radiation whose frequency is equivalent to the energy difference between the two levels. Since 1960 the field of NMR has seen an explosive growth which started with the development of pulsed Fourier-transform NMR and multidimensional NMR spectroscopy and still continues today.
The limitations of NMR spectroscopy result from the low inherent sensitivity of the technique and from the high complexity and information content of NMR spectra. These problems are partially alleviated by new developments: The sensitivity and resolution of NMR are increased by progress in spectrometer technology. Progress in the theoretical and practical capabilities of NMR lead to a increasingly efficient utilization of the information content of NMR spectra. Parallel developments in the biochemical methods (recombinant protein expression) allow the simple and fast preparation of protein samples. Heteronuclei like 15N, 13C and 2H can be incorporated in proteins by uniformly or selective isotopic labelling. Spectra from these samples can be drastically simplified. Additionally, some new informations about structure and dynamics of macromolecules can determined with these methods.
All these developments currently allow the structure determinaton of proteins with a mass of up to 30 kDa. Spectroscopists hope to extend this limit to even larger values (perhaps 40-50 kDa) by further improvements  . Recently, a group reported the backbone assignment of a protein complex with a mass of 64 kDa .