Transnational organised crime at sea is a growing international concern. However, and despite its importance, the concept remains uncertain and contested. Debate continues over the term itself, what illicit activities it incorporates and excludes, and how these can be meaningfully conceptualised in ways that that both recognise the diverse nature of the concept yet also provide a basis for an integrated response to the challenges it presents. In this paper, we provide a systemic conceptualisation by proposing the concept of blue crime as a framework. Our goal is to provide a firm basis for future enquiries on the different types of blue crime, to trace their distinct characteristics and identify how they intersect, and to consider what kinds of synergies can be built to respond to them.