In this chapter we discuss the challenge that the regulation of greenhouse gas (GHG) emissions poses to the mandate and structure of the International Maritime Organization (IMO). The discussion is undertaken against the backdrop of the evolution of the organization since it was established. We consider the implications of the IMO Initial GHG strategy adopted in 2018 in paving the path to the international shipping’s contribution to mitigation in response to the Paris Agreement, 2015. The long-term decarbonization of shipping constitutes one of the greatest challenges the IMO has faced since it first embraced the environmental mandate in the late 1960s and early 1970s. We argue that the IMO may have to consider structural and functional change to meet the demands of the strategy and in particular with respect to a possible market-based measure.