Abstract: Ever since Martin Luther King Jr. asked whether we will be extremists for the preservation of injustice or for the extension of justice, scholars and political activists have been tempted by the idea that there is nothing wrong with extremism per se, only with particular forms of extremism. This lecture will examine this idea with a sceptical eye. Apologists for extremism rely on overly bland and relativistic conceptions of what it entails. This lecture will make the case for a more substantive and absolutist conception of extremism as an ideology and as a political methodology. On this more demanding conception of extremism, it is much less obvious how it can be a force for good.