Macromolecular crystallography was preceded by the older field of small-molecule x-ray crystallography (for structures with less than a few hundred atoms). Small-molecule diffraction data extends to much higher resolution than feasible for macromolecules, and has a very clean mathematical relationship between the data and the atomic model.  The residual, or R-factor, measures the agreement between the experimental data and the values back-calculated from the atomic model.  For a well-determined small-molecule structure the R-factor is nearly as small as the uncertainty in the experimental data (well under 5%).  Therefore, that one test by itself provides most of the validation needed, but a number of additional consistency and methodology checks are done by automated software as a requirement for small-molecule crystal structure papers submitted to the International Union of Crystallography (IUCr) journals such as Acta Crystallographica section B or C. Atomic coordinates of these small-molecule structures are archived and accessed through the Cambridge Structural Database (CSD) or the Crystallography Open Database (COD).The first macromolecular validation software was developed around 1990, for proteins. It included Rfree cross-validation for model-to-data match, bond length and angle parameters for covalent geometry, and sidechain and backbone conformational criteria.  For macromolecular structures, the atomic models are deposited in the Protein Data Bank (PDB), still the single archive of this data.  The PDB was established in the 1970s at Brookhaven National Laboratory, moved in 2000 to the RCSB (Research Collaboration for Structural Biology) centered at Rutgers, and expanded in 2003 to become the wwPDB (worldwide Protein Data Bank), with access sites added in Europe ([1]) and Asia ([2]), and with NMR data handled at the BioMagResBank (BMRB) in Wisconsin.