This chapter investigates the role of state involvement in the decommissioning of nuclear power plants. Decommissioning nuclear reactors remains a complex and costly endeavor, with only a fraction of closed reactors having completed the process. Utilizing a theory-informed explanatory case study approach grounded in transaction cost economics, this research analyzes how governance structures impact project efficiency and effectiveness in the United Kingdom and the United States. The first case demonstrates a dominant role of the state in conducting and overseeing decommissioning efforts, while in the United States, the state primarily acts as a regulator, allowing the private sector to lead initiatives. By exploring the strengths and weaknesses of these different governance modes, the analysis highlights the critical interaction between technological and organizational challenges in nuclear decommissioning, ultimately providing insights into the economic and operational frameworks necessary for future projects.