Landmark developments in autonomous vessel technology have the potential to deliver economic, environmental, and security benefits on the world’s oceans. Tempering the promise of that future is the stark reality that maritime autonomous surface ships (MASS) challenge the existing international order of the seas. This article examines the promise and perils of MASS in three areas of enduring significance to commercial vessels, naval forces, and industry regulators: search and rescue (SAR), maritime counterdrug operations, and navigational safety. This article concludes that autonomous vessel technology will lead to a superior global regime for maritime SAR operations, enhanced detection and interdiction of maritime drug traffickers, and greater navigational safety. At the same time, it argues that embedded within each of these benefits are formidable challenges. Specifically, the duty to render assistance at sea has yet to be defined in the context of MASS operations, especially with regard to fully autonomous vessels. In the context of maritime drug trafficking, autonomous vessels will inure to the benefit of both law enforcement entities and the criminal enterprises they seek to interdict. Finally, while autonomous technologies may one day render ocean navigation a safer enterprise by reducing human error, it is not yet clear if and how existing instruments—which presuppose human crews—can be applied to MASS. These challenges are not insurmountable. However, their adequate resolution will require the same ingenuity and resolve that has brought us to the threshold of an autonomous future at sea.