One of the centrally contested concepts in the philosophy of science. The phrase suggests the dictate or fiat of a lawgiver, and for most thinkers, at least until the 18th century, discovering the laws of nature would be discovering how God had ordained that events should unfold. Without that backing the notion may seem to disappear, leaving only a conception of nature as a succession of different events that just happen to show patterns and regularities: ‘just one damn thing after another’. To this can be added the view that the human mind, contemplating the regularities in events, selects some as reliable and adopts them as fixed premises for purposes of prediction and action: these we call laws. This is in effect the approach of Hume. It gives rise to a programme of describing just which regularities are ‘lawlike’ or fit to be selected for this status. Suggestions include those which are universal, simple, or contain the right kind of vocabulary, or fit with our other beliefs in various ways. Realists about laws of nature insist that no such approach gives us enough. They demand a more substantial or robust conception of laws as real features of the world: a kind of stabilizing glue or straightjacket, ensuring that natural events not only happen to fall out as we find them, but must do so. The problem for realism about laws of nature is to make sense of this necessity. It seems to transcend experience, since the methods of natural science seem adapted to showing us only how things do happen, not how or why they must happen in the ways we find.