Constitutional interpretation is problematic because it can be difficult to distinguish legitimate interpretation from illegitimate change. The distinction depends largely on what a constitution is. A constitution, like any other law, necessarily has a meaning, which pre-exists judicial interpretation: it is not a set of meaningless marks on paper. Any plausible constitutional theory must offer an account of the nature of that meaning. In doing so, it must address two main questions. The first is whether the meaning of the constitution is given solely by its words, or also by additional evidence of underlying intentions, purposes or values. The second question is whether, in either case, its meaning is determined by the original meanings, intentions, purposes or values of the founding generation, or by contemporary meanings, intentions, purposes or values. This question divides so-called ‘originalists’ from ‘non-originalists’. This article discusses all these alternatives, and the main arguments for and against them. It is subtitled ‘Originalism’ because it defends a moderate version of that position; however, it is argued that this is equivalent to a moderate version of non-originalism.