This paper deals with a few aspects of recently developed new NMR technique, two-dimensional (2D) NMR, ranging from its principle to its biological applications. First the historical background of the new technique is surveyed and its underlying principle is explained. To cover the versatile applicability and flexibility of the technique in the NMR. Studies, then, various experimental techniques proposed up to now are classified from the general point of view of this new experimental frame. In Sec. III spectroscopic characteristics experienced in the practical data handling are described. We observe there that the causality principle and several theorems of multidimensional Fourier transformations play a key role to costrain the appearance of 2D spectra in general. Applications of 2D NMR to the protein study are described in the following sections. For the successful use of NMR techniques in the elucidation of protein structures in solution, the individual resonance assignments for each of the amino acid residues are a crucial first step. The combination of two classes of 2D NMR experiments, J-resolved and correlated 2D NMR, was found to make it feasible to obtain this critical data in a systematic way. Based on the individual assignments of C alpha H and C beta H proton resonances in the basic pancreatic trypsin inhibitor (BPTI), coupling constants, 3J alpha beta, between C alpha H and C beta H and their chemical shifts were determined with 2D J-resolved NMR. The results were compared with the values of 3J alpha beta predicted from the crystallographically determined structure with a Karplus-type curve Flexibility about C alpha-C beta bonds in BPTI was then discussed for the individual residues. A model to explain the difference in internal mobility of the protein recognized between the NMR results and the prediction from the X-ray structure was also proposed. On this model NMR parameters were compared with a theoretical parameter, residue accessibility, calculated from the X-ray structure. With a few exceptions, most of amino acid side chains in the interior of the protein were found to be locked into unique spatial orientations, with the mobility restricted to rapid torsional fluctuations about a unique chi 1 value. For the residues on the protein surface structural rearrangement was found which includes rapid averaging between two or several distinct populated values of chi 1. As an extension of the picture for the protein flexibility in the solution, the thermal expansion coefficient of the protein molecule was estimated with the NMR chemical shift data. The result has characterized the structural aspect of the protein molecule, that is the protein is not strictly rigid in the native state and has a liquid-like property in the solution phase.