A brief outline of the quantum physical basis of ¹H nuclear magnetic resonance (NMR) spectroscopy is provided. ¹H NMR is the pre-eminent method for the qualitative structural analysis of organic molecules. Its potential for quantitative analysis, although recognized soon after ¹H-NMR became commercially available, has recently become a focus for practical applications. Methods for the assignment of the purity of individual organic compounds based on NMR spectroscopy are now widely implemented for this purpose by National Metrology Institutes (NMIs) to underpin their in-house capabilities for the characterization of primary calibrator materials. Progress in a joint BIPM-NMIJ collaboration to develop and support the metrological basis of qNMR methods for purity assignment is described. This collaboration has investigated and validated experimental parameters for acquiring ¹H-qNMR spectra and identified a suite of seven Internal Standard Reference Materials (ISRMs) that provide a universal set of calibrators for use in internal standard qNMR. Examples are described of the application of qNMR to the assignment of the mass fraction content of mycotoxin and peptide materials that were required to support the BIPM Organic Analysis Work Program. These purity assignments would not be practicable by the traditional mass balance approach. There are inherent limitations in the resolution achievable by NMR when applied to structurally complex compounds. This meant that in order to assign the content of the specific analyte for these complex compounds the raw qNMR result, which provided an assignment of the “total” organic content in the material, required an additional correction for the related structure impurity content of the material determined by a high-resolution LC-MS/MS method. Finally recent developments reported in the general literature of the use of advanced multi-dimensional NMR techniques to investigate the higher-order structure of large, complex proteins are illustrated by an example of their application in the characterization of the higher-order structure of monoclonal antibodies.