This thesis is a defence of constitutional history – a once prosperous field of study in the UK, but now fallen into disrepute and neglect. The first question addressed by this thesis, therefore, is: Why has constitutional history fallen from favour? It is argued that it is principally due to a fundamental and widespread misunderstanding as to the nature of constitutions and laws, and how these things change over time. Moreover, it is due to the field’s association with a group of nineteenth-century writers, elements of whose approach and assumptions have – rightly – come to be regarded as outmoded and distasteful. These things need to be put aright if constitutional history is to be revived. Certainly, there has been some movement to resurrect the field, but this is unlikely to be successful or profitable without a solid underlying constitutional theory. This is still lacking. This thesis, therefore, seeks to provide that solid underlying constitutional theory. It does so by suggesting an interpretative framework for constitutional theory and history in Part I. This consists of three theories: the Theory of Constitutional Ubiquity; the Associational Theory of Law; and the Generational Theory of Law. These are complemented by a chapter on the theory of succession and two appendices. Together, these argue that all societies – regardless as to time or place – have constitutions, which can and do change over time. Consequently, there is no reason why constitutional history should be disregarded. That it has been is regrettable. Parts II and III seek, firstly, to show the framework’s validity and explanatory power, and, secondly, to show how constitutional history might look in the future. They do this by considering constitutions in, primarily, mediaeval thought and practice – the mediaeval period being a time in which the existence of constitutions is often dismissed or, at least, viewed with considerable scepticism. Part II, concerning mediaeval thought, argues that people throughout the mediaeval period were primed to think about constitutional matters. Part III considers mediaeval and early modern practice through the lens of succession and governance vis-à-vis the royal minorities of mediaeval England. It argues that contemporary practice shows clear regard for what can rightfully be called a constitution, though it changed from generation to generation. Having charted constitutional history’s rise and fall, and having made the argument for its resuscitation based on the principles set out in the framework, the thesis concludes by considering constitutional history’s future. There is, or seems to be, cause for optimism.