The International Maritime Organization (IMO) is working on adopting a greenhouse gas (GHG) emissions pricing mechanism to drive the decarbonization of international shipping. This chapter analyzes key reasons why the GHG pricing mechanism remains one of the most contentious points of debate in the negotiations. The chapter highlights that GHG pricing mechanisms can be used to achieve different aims (e.g., flexibility, emissions reductions, revenue raising). Differences in proposals under consideration at the IMO seem based at least in part on a preference for one aim over another. These differences in views on what GHG pricing should aim to achieve seem to be at the core of the disagreement between states. In addition, there are various (potential) challenges related to implementing an IMO GHG pricing mechanism. This chapter looks at both legal (compatibility with MARPOL and WTO law) and non-legal challenges (e.g., impacts on states, revenue use, and the availability of alternative fuels) and discusses some potential ways forward.