Criminalisation is now seen as one of the central issues in criminal law theory. However, in spite of the (rather belated) recognition of the importance of the topic, a number of fundamental questions remain unaddressed (Duff et al. 2014) and, in many cases, not even openly acknowledged. I discuss three such questions here, prompted by the articles in this excellent special issue on ‘Hidden Criminalisation’ and, in doing so, I raise a fourth and broader issue about the possible limits of criminalisation theory.