As uncrewed maritime vessels and the technology supporting them advance, they will play an increasingly important role in hybrid threat operations and in ways to counter them. This Working Paper analyzes the current and potential impact of uncrewed vessels in the context of hybrid threats. It first reviews the history of uncrewed surface and undersea vessels, followed by a description of how these technologies can be employed in hybrid threat operations, illustrated through a case study of a hybrid threat scenario in the Baltic Sea. Finally, the paper assesses Chinese, Russian and Iranian state-of-the-art uncrewed maritime vessel capabilities.