There is an argument often deployed by those who object to the rapid advances in technology, whether in agriculture and animal husbandry or in medicine, that some procedure is ‘unnatural’, and therefore should not be actually prohibited. An attempt is made to analyse and appraise the moral force, if any, of the dichotomy ‘natural’/‘unnatural’, especially in the area of assisted conception. The emotional resonances of the concept of Nature are partially explored, and found to be deep-seated and various, but not of themselves the source of moral imperatives.