Our understanding of the function of protein molecules was revolutionized in the 1960s by the use of X-ray crystallography to give a three-dimensional picture of their structures at atomic resolution. The structure of myoglobin was rapidly followed by the structure of several hydrolytic enzymes such as lysozyme, carboxypeptidase, ribonuclease, chymotrypsin, and subtilisin; and, not long after, by the much more complicated structure of haemoglobin, composed of four myoglobin-like molecules interacting with each other. The first hydrolytic enzyme structures showed us how enzymes perform biological catalysis by immobilizing their substrates at the enzyme active site, and gave us definite ideas about the specific functions of different parts of the protein molecules. These ideas had to be treated as hypotheses, because there was no direct method to check them. A few particular points could be proved by cunning but tedious experiments.