This chapter offers a brief characterisation of mediatisation research, and critically discusses mediatisation as a “sensitising concept”, as a research programme, a research field, an approach, or a paradigm. It provides a historical overview of how the concept and the field has developed over the past few decades until now, and what has shaped it in this way. The chapter aims to account for the various approaches (the institutional perspective on mediatisation, the technological, and the cultural or social constructionist); and discusses ontological and epistemological differences. It describes the main challenges and objections/criticisms to mediatisation studies defined so far, types of transformations that have been analysed, and which are neglected and marginalised. The chapter also presents an overview of the key concepts discussed in this book.