In his new book What is a Law of Nature?, David Armstrong gives an account of functional laws on the basis of the theory, originally proposed independently by Armstrong himself, Dretske, and Tooley, and further developed in this work, which asserts that laws are relations of necessitation between properties. On the theory, properties and relations are universals, and so a law is a relation between universals and is itself a universal. There are two reasons why Armstrong's account of functional laws is worth discussing. First, any theory that purports to be about laws that are investigated by science–I assume that we are only interested in such theories–must cover functional laws and not merely be adequate for generalizations about ravens or grue. Second, Armstrong claims that the main rival to his theory, the regularity theory of laws, cannot give a satisfactory account of functional laws (p. 22 and p. 37, all references to What is a Law of Nature? unless otherwise stated). If this is true, and if Armstrong's theory can provide a satisfactory account of functional laws, then this may incline us towards his point of view.