Thalidomide comes in two forms: a left-handed compound which is a powerful tranquilizer, and a right-handed version which can disrupt fetal development causing severe handicap. As a necessary consequence of synthetic methods available in the early 1960s the two forms were present in equal proportions in the manufactured drug, with catastrophic consequences. The thalidomide story is perhaps the most painful reminder of the importance of stereochemistry—the spatial ordering of groups in a molecule can be as influential as the chemical nature of the groups themselves. A principal problem in organic synthesis is, therefore, the development of methods for producing complex molecules with a stereochemically defined structure.