A technique employed by chemists and structural biologists to determine the atomic structure of molecules and proteins. Sometimes called X-ray diffraction, the technique requires that one is able to grow crystals of your pure compound that are of reasonable size and quality. Obtaining the crystal itself is often the hardest part of this process. To induce a molecule to crystallize, one often uses mixtures of solvents, salt conditions, other organic compounds in order to find the optimimum growing conditions.

Once an adequate crystal has been grown, you then take the crystal and place it in a X-ray diffraction instrument. An X-ray source such as a synchotron shines an intense beam of X-rays on the crystal. Depending on the way the x-rays are bent and dispersed by the crystal (diffraction), they make a pattern on a photographic plate or sensitive detector placed after the crystal. This pattern is then analyzed to determine the atomic structure of the compound in question.

X-ray crystallography and NMR are the two techniques currently available which can provide atomic detail for molecules such as proteins. A database of protein structures can be found in the Protein Data Base or PDB and smaller organic compounds can be found in the Cambridge Structural Database. Knowing the atomic level structure of a protein is a huge step towards understanding how it functions in the cell.