Since the publication of Rethinking Industrial Relations in 1998, John Kelly's mobilization theory has been used by many scholars attempting to understand union organizing-often using specific campaigns to unpack and analyse various elements of the theory that help to explain the success or failure of collective mobilization. We review this literature and highlight the major areas of interest from the book-injustice, framing, and leadership-and the contribution to industrial relations research. We find that the terms mobilizing and organizing are used interchangeably, which, we argue, is problematic and has led to confusion about what is actually happening in unions. Unpacking the difference between the two terms can help to explain limitations in the way mobilization theory has been used by scholars, and, at the same time, deepen our understanding of why unions have not been particularly successful in reversing their decline despite the 'turn to organizing' over the last few decades.