Environmental governance approaches increasingly include non-state actors such as industry and NGOs to promote and implement voluntary private sustainability standards. This type of private governance is particularly evident within the fisheries sector. Alongside the rise of certifications and ecolabels for sustainable seafood, a new type of market-based incentive called Fishery Improvement Projects (FIPs) has been developed to meet the growing demand for sustainably sourced seafood. By employing a multi-stakeholder approach, these projects aim to use the power of the private sector to improve fishery management and fishing practices. FIPs have grown in number and have become a key feature of the global seafood landscape, but they have only just started being studied. The aim of this thesis is to understand the potential and challenges for private governance approaches as pathways towards more sustainable fisheries by using FIPs as a case study. Paper I contributes with the first global systematic description of FIPs governance processes by examining reported actions, the actors involved, and their achievements in terms of policy and practice outputs, based on a sample of 107 FIPs. The paper shows that the most common policy outputs of FIPs were new management plans and management bodies. Practice-related outputs were dominated by gear changes, and observer and traceability programs. It also shows that FIPs include a diversity of actor types, but that fishers and retailers were relatively absent. By using theories from the private governance literature, the paper suggests how FIPs strategies and outputs can be seen as complementing or strengthening governmental interventions. Paper II contributes with an in-depth case study of the blue swimming crab FIP in Indonesia. By using the lens of institutional entrepreneurship, the study explores why and how industry actors aspire to change the fishery, and their outcomes throughout the value chain, from importers in the US to village fishers in Indonesia. The paper finds that, although some industry actors possess leverage to foster several improvements including procurement specifications and new fishery management policies, the local social-ecological complexity makes it difficult to achieve behavioral changes among fishers and traders on the ground. The FIP has therefore had limited outcomes. The paper highlights the need for FIP strategies to include local context. The paper expands on the theoretical understanding of institutional entrepreneurship in social-ecological systems. Together these two papers contribute with new empirical understanding as well as theoretical advancement to the scholarly debate of private governance and fisheries sustainability.