This chapter outlines an approach to the study of categorisation in language practice grounded in the work of American sociologist Harvey Sacks known as Membership Categorisation Analysis. MCA proposes that categories and their associated ‘normative orders’ are not simply stored in people’s heads; they are used as part of the accomplishment of practical tasks in various social settings. Thus, MCA focuses on how categories are used to do things within talk and text, such as criticising, complaining, praising, encouraging, inviting, commending, blaming, and so on. Moreover, we also propose that power relations can be central to the study of categories. We demonstrate this in a political context through analysis of a political speech made by the previous British Prime Minister, Theresa May. We show that May’s category-based reasoning about social injustice was used to appeal to a sense of social solidarity and moral responsibility of ‘the fortunate’ to help the ‘less fortunate’ in society. We conclude that MCA has value for students and practitioners of language because no other approach comes as close to the study of how categories are used in talk and text in real-life situations.