X‐ray crystallography is the prime technique for determining the 3D structure of chemical and biochemical compounds, providing the conformation of the molecules in the crystal in addition to their configuration. The use of X‐rays with wavelengths around 1 Å means that the positions of individual atoms in the molecule can be defined. However, the technique needs a crystal to amplify the signal from a single molecule, and crystal growth requires the compound under study to be homogenous and that all the molecules take up a single stable conformation in the crystal, which restricts crystallography to the study of long‐lived states. In small molecule structures, the molecules are closely packed with few solvent molecules present. In contrast, crystals of macromolecules contain on average roughly 50% aqueous solvent and as result are less well ordered, with the resolution of the experimental X‐ray data being less than atomic. Thus, while for small molecule structures there are sufficient experimental observations to provide accurate atomic positions, the more limited resolution of macromolecular structures means that the X‐ray observations must be supplemented by stereochemical restraints.