Radicalism is a term well-entrenched in the historian's lexicon. A search on the Royal Historical Society's on-line bibliography for British and Irish history retrieves nearly 300 books, essays and articles that use the word in their title. The total rises to nearly 850 if the search term used is ‘radical’ rather than ‘radicalism’. Radicals and radicalism are everywhere, at least from the sixteenth century onwards. They come in all sorts of varieties, popular and elite, of the left and of the right, Tory and Whig; British, English, Irish, Scottish and Welsh. But the word is, in many of its uses, curiously weak. We are likely to have a rough idea of the sorts of things that might be meant by calling someone a socialist, a conservative or a liberal, and a corresponding sense of what the equivalent ‘isms’ might look like, even if that sense quickly becomes complex and sub-divided. But what sense do we get from hearing someone described as a radical? We would assume that the ‘socialisms’ of different periods might show some resemblances (however forced or artificial), and there is a recognisable core meaning in describing Thomas More, Gerrard Winstanley and Karl Marx as communists. All of them envisaged an ideal society in which private property was abolished. They have all been described as radicals too, but it seems less clear what this label tells us about them.