This thesis concerns: (a) how we should understand legislative words and phrases if they have changed in meaning since they were enacted; and (b) how the UK courts have actually done so. The key question is: should we give legislative language its original meaning (treating its meaning as 'static'); or, if not, should it be given its current meaning (treating its meaning as 'mobile')? I claim that, to address this 'temporal issue', we need to recognise that legislation is generally 'always speaking'. This means that we are to treat it as if it were being constantly re-enacted, and therefore to treat is as if it is able - like, say, traffic signs - to 'speak' to new facts and situations as they arise, on an ongoing basis. But this raises a key question: does legislation necessarily 'say' the same thing each time it 'speaks'? The 'always speaking' metaphor suggests we should give legislation its current meaning; but this creates certain problems, particularly for old legislation where the meaning of words or phrases may have changed radically since the time of enactment. I suggest a two-fold response to this temporal issue. First, that whether legislative language should be given its original meaning (or, if not, it should be given its current meaning) should turn on what the original enacting legislature objectively intended on that question. I call this 'intention-dependence'. Second, that we need to distinguish three notions of meaning that can (and do) differ in mobility. The first notion of 'meaning' is application - roughly, the things that the legislative words apply to at a particular time - and it is typically mobile. A common response to the temporal issue is to distinguish application from meaning, and to give legislation its original meaning but its current application. This response, I argue, is inadequate: because 'meaning' (as opposed to application) conflates two further notions which legislatures often intend to differ in their mobility. The second notion ('character') is the abstract, purely linguistic, meaning of words: the notion of meaning that dictionary definitions attempt to capture. We will usually best understand what someone meant by what they said if we give their words their original character. For example, Queen Anne said of the newly-built St. Paul's Cathedral that it is "awful, artificial, and amusing". To understand that she was complimenting, rather than criticising, the Cathedral's architects we need to travel back to 1711: when those words meant 'awe-inspiring', 'artistic', and 'thought-provoking'. The third notion ('content') is a more concrete, practical, notion of meaning: it is what tells us what a word or phrase actually applies to. Consider a law made in 1791 that forbids 'cruel punishments'. The character of 'cruel' is much the same now as it was in 1791, but what counts as being cruel today is very different from what counted as cruel in 1791 (when flogging and ear-notching, for example, were common punishments). We seem to want to give this law its current content - so that, today, it forbids flogging and ear-notching - rather than its original content. In Part I of this thesis, I articulate these three notions of meaning, show their utility in modelling adjudication, and relate them to three types of uncertainty in meaning: vagueness, context-sensitivity, and ambiguity. I show that this tripartite meaning framework admits of four general approaches to the temporal issue, depending on which notions of meaning are static or mobile. I then address some questions about these approaches: when a legislature might use concept-words referentially, and whether the original legislature's expectations as to how a provision will be applied are binding. Finally, I review some reasons for or against adopting these approaches, and I suggest that they most strongly favour the approach that treats legislative content (and application) as mobile but treats its character as static - which I call a 'Character Originalist' approach. In Part II, I turn to identifying the UK response to the temporal issue. I start with the historical (pre-1997) UK approach. I argue that the standard account of this is flawed: as the two doctrines on which that account is based do not establish what they are standardly taken to establish, and UK courts have treated the content of legislative language as mobile in appropriate cases. I then turn to the modern (1997-) UK approach. I analyse Lord Steyn's landmark judgment in Ireland (1997) and I show that it established two principles: (a) intention-dependence; and (b) what I call the 'Mobility Presumption'. I explore the sources of Ireland, and its aftermath, to identify which notions of meaning we are to presume to be mobile; and I hypothesise that the UK approach is, generally, Character Originalist. Finally, I test that hypothesis against the UK case law: and I conclude that there is strong support for the hypothesis. The meaning of UK legislation may therefore, generally, evolve over time - but only within the bounds set by its original character. So the solution to our puzzle is to recognise the sense in which we must give legislation its original meaning (character) and the sense in which we must give legislation its current meaning (content).