I propose an argument for the thesis that laws of nature are necessary in the sense of holding in all worlds sharing the properties of the actual world, on the basis of a principle I propose to call the Causal Criterion of Reality (CCR). The CCR says: for an entity to be real it is necessary and sufficient that it is capable to make a difference to causal interactions. The crucial idea here is that the capacity to interact causally - or to contribute to determining causal interactions - is not only the ultimate metaphysical ground for the existence of an entity, but it also provides a criterion for determining the nature of that entity, i.e. its properties. The alternative is to conceive of laws of nature as contingent: they could be different from what they are like in the actual world, where that possibility is understood to be metaphysical, not only epistemic. For the sake of this paper, I shall accept Armstrong's (1983; 1997) thesis that laws of nature are relations between universals. I also follow Armstrong in the view that both the existence and the properties of particulars are metaphysically independent of the existence and identity of other particular. However, what is controversial and what I shall challenge is his thesis that universals are like particulars in the following respect: according to Armstrong, each universal is a logically distinct entity whose existence and identity is independent of the existence and identity of other universals. My aim in this paper is to show that the identity of a universal is entirely determined by its lawful relations to other universals. The crucial premise I use is the thesis that the CCR is a universal criterion, which applies both to particulars and universals. From the thesis that the identity of a universal is exclusively determined by laws, it follows that laws are necessary in the sense that they cannot differ without the universals they link also being different. This creates a difficulty for those authors who, as Armstrong, accept the CCR but nevertheless defend the view that laws are contingent.