This essay examines the obligation of Flag States to exercise effective control over vessels flying their flag, ensuring that they comply with international standards and regulations. The main objective is to highlight the importance of due diligence in the, accounting and maritime context, both for maritime safety and for the protection of the environment and the well-being of the crew. The results show that due diligence includes measures such as regular monitoring of ships, the issuance enforcement of national and international regulations, and cooperation with other States and international organizations. In addition, the need to verify compliance with Labour and environmental standards set by organizations such as the International Labour Organization (ILO) and the International Maritime Organization (IMO) is underlined. Key findings indicate that due diligence in flagging ships is crucial to prevent maritime accidents, protecting the marine environment, and ensure fair working conditions. In addition, due diligence is an essential tool for to prevent illicit activities such as money laundering and the terrorist financing in the maritime sector. In short, due diligence is not only a legal obligation but also an ethical principle that reflects the commitment of States to the safety, sustainable development of shipping, and world peace.