This chapter argues that the conflict at sea was an important and frequently overlooked part of the Napoleonic Wars. Focusing primarily on the Royal Navy and French maritime forces, but also mentioning the navies of Spain, Denmark, Sweden, the Netherlands and United States, it outlines the manifold ways in which maritime warfare shaped wider events on land, and helped determine the conflict’s final outcome. It demonstrates that French attempts to invade Britain were successfully rebuffed by the Royal Navy, ensuring that Britain remained in the conflict. The chapter then offers a more modern take on the commonly misunderstood Battle of Trafalgar, arguing that it was far from decisive and did little to change the course of the war. The naval conflict continued in earnest after 1805, and the war of trade became all-consuming, particularly after the inception of ‘Napoleon’s Continental system’. Here the navy offered a stubborn resistance to the French Emperor’s objectives, helping to encourage illicit trade with the European continent while also expanding Britain’s empire and mercantilist reach elsewhere in the globe. Finally, it demonstrates that maritime support was crucial to the land war, not least Wellington’s Peninsula campaign.