These essays deal with questions of navigation and, more broadly, the intellectual challenges posed by Spain’s acquisition of an empire across the Atlantic. Crudely, they had to find out what was where and how to get there. The first section of the volume looks at the 16th-century Sevillan cosmographers and pilots charged with this task: their achievements, the social and political context in which they worked, and the methods used to establish scientific truths - including the resort to litigation. Ursula Lamb then turns to examine specific problems, from the routing of transatlantic shipping to the application of cartographic coordinates to allocate unexplored territories. The final articles move forward to the time when, after a lapse of two centuries, Spanish nautical science became revitalised, and the Spanish Hydrographic Office was established.