Proanthocyanidins (PACs) are a structurally diverse and complex group of condensed, oligomeric flavan-3-ols that are widespread in woody and some herbaceous plants. PACs contain several subtypes, such as procyanidins, prodelphinidins, and propelargonidins based on the corresponding basic units catechin and epi-catechin (syn. epicatechin), gallocatechin and epi-gallocatechin (syn. epigallocatechin), as well as afzelechin and epi-afzelechin (syn. epiafzelechin). B-type PACs have one interflavanyl bond originate from coupling C-4 with one of the nucleophilic C-6 or C-8 of a subsequent monomer. A-type PACs have an additional ether bond between C-2 in the "top" and an A-ring hydroxyl moiety in the "bottom" unit. Since their discovery in 1947, PACs have attracted scientific attention for both their complicated structures and medicinal value. While PACs have been associated with a broad spectrum of bioactivities, including antioxidant, anticancer, antiinflammatory, antidiabetic, 7 neuroprotective 8 effects, and play an important role in the fields of nutrition and health. The focus of our research at the chemistry-dentistry interface has been the exploration and understanding of their structure-specific abilities to enhance the biomechanical properties of dentin, the soft collagenous tissue of teeth. The structural complexity of PACs increases exponentially with their degree of polymerization (DP), the linkage (A-and B-type) and absolute configuration of their interflavanyl linkages (IFLs), as well as the combination of the possible stereoisomeric monomers. Additional analytical challenges are posed by their formation of atropisomers, which interferes with chromatographic separation and often makes NMR spectra acquired at room temperature uninterpretable due to exceedingly broad lines. Additionally, in some cases, resonances can be "missing" because of H-D exchange in deuterated NMR solvents, even for hydrogens that might initially be considered "not likely to exchange". Collectively, these properties make the structural elucidation of trimeric or higher oligomeric PACs an often challenging undertaking. In general, reliance on literature data is hazardous as structure-related publications seldom report full assignment of the H and 13 C NMR data. This provides grounds for questioning the ultimate reliability of at least some PAC structures and their spectroscopic assignments and rationalizes the importance of developing highly cohesive data sets.