This chapter outlines the governance landscape of ocean acidification (OA), identifying thirty-one actors and twenty-seven instruments at the international, regional, local, and transnational levels. The map indicates that no single institution leads OA governance; rather, multiple institutions address different aspects through implicit mandates rather than explicit ones. Only the Ocean Acidification Alliance has a clear OA mission. Most institutions operate across various fields such as climate, biodiversity, and marine protection, resulting in functional overlaps without a clear hierarchy. Applying regime complex theory, the chapter contends that OA governance constitutes a regime complex – partially overlapping, likely non-hierarchical institutions governing the same issue area. Three factors affirm this classification: divergent interests (economic vs. environmental), high uncertainty (scientific complexity and multiple actors), and limited linkages between institutions. The regime complex is situated at the intersection of climate, marine, and atmospheric governance systems. This characterisation is vital, as it implies that global experimentalist governance could effectively utilise these existing institutional arrangements rather than supplanting them, laying the groundwork for selecting suitable governance approaches in subsequent chapters.