Structural biology has transformed a number of fields of biochemistry and biology. A few examples are the following. The allosteric uptake and release of oxygen from haemoglobin obtained an elegant description from the work of Perutz [Perutz (1970), Nature (London), 227, 726-739; Perutz et al. (1998), Annu. Rev. Biophys. Biomol. Struct. 27, 1-34]. The structure of tRNA highlighted the fact that the two functional facets of the molecule, the anticodon and the 3'-end, which is charged with amino acids, were 75 A apart [Robertus et al. (1974), Nature (London), 250, 546-551; Kim et al. (1974), Science, 185, 435-440]. This had interesting consequences both for charging by tRNA synthetases and for the role of tRNA as the adaptor in protein synthesis on the ribosome. The structure of the ATP synthase illustrated how the enzyme could hydrolyze or synthesize ATP by a rotating mechanism [Abrahams et al. (1994), Nature (London), 370, 621-628]. This transformation is a consequence of the fact that such structural insights on different levels and of different characters can provide a basis for the interpretation or reinterpretation of old observations. In addition, the design of experiments from a structural basis has become more relevant and focused.