This dissertation explains how industry actors influence environmental maritime regulation in the International Maritime Organization (IMO). The reason for this topic and focus is the significant implications for the role of private actors in global regulatory affairs, coupled with the relative lack of explanations concerning how industry actors actually change political outcomes when they participate as political discussants. In the IMO, industry actors have extensive access to policy development, which makes it a relevant case to understand in more detail. The theoretical basis of the dissertation is rooted in organizational institutionalism, but the foundation for the issue and its relevance is drawn from International Political Economy (IPE) literature. One novelty of the use of organizational institutionalism is the perspective it offers in terms of understanding the way IMO deliberations play out. Core concepts are institutionalized norms, values, and taken-for-granted beliefs, which together serves as the base for explaining the power of industry actors in the IMO and the way these actors exercise influence. Methodologically, the dissertation approached this issue through direct participation in IMO sessions and the use of interviews with IMO delegates from 2016 through 2018, which includes almost 300 hours of observation and more than 60.000 words of field notes. The material was analysed qualitatively by using process-tracing, which allowed the inference of the most plausible explanation of how industry influence works. The findings of the dissertation shows that industry actors gain influence by deploying technical arguments to influence substance or appeals to consistency to influence format of the regulation. State delegates and other industry delegates consider the use of technical arguments to be legitimate, because IMO delegates fundamentally view the IMO process as one of solving technical problems and making global standards rather than a political process. Industry achieves influence when state delegates believe the reasoning and substance of the technical arguments makes sense, as long as state delegates believe the issue under discussion is not too political to allow industry influence. This results in a constant balance, where state delegates weigh the political contention against the potential contribution of industry actors in a given discussion. One important implication of this is the role of ‘invisible rules’, or institutionalized norms and beliefs, in the structuring of industry influence. Industry power is both constrained and enabled by beliefs and norms that IMO delegates’ largely take for granted, rather than formal rules or procedures that protect the IMO from capture by private interests. This dissertation and its findings add to the theoretical understanding of industry power in global governance and international regulation by showing how industry influence pans out in a specific case, and expanding the theoretical repertoire for how researchers can approach such challenges. It also adds to the discussion about the appropriate role of firms and business interests in political life, and shows that there are nuances in the way industry power can be controlled and misused in an intergovernmental organization.