Following recent historiographical appeals on the need to study knowledge exchanges between European maritime rivals and their impact on overseas expansionist processes, this book makes this study for the Portuguese overseas empire between the fifteenth and seventeenth centuries. As the first European maritime power to systematically launch long-distance voyages, Portugal became a model worth emulation when Spain, France, England and the Dutch Republic started their own overseas enterprises. In different chapters that each adopt a case study relation (Portugal-Spain, Portugal-England, Portugal-France and Portugal-Dutch Republic), this book documents how Portuguese maritime knowledge was outsourced by its maritime rivals. The impact that Portuguese nautical knowledge had is evaluated, resorting particularly to a wide range of diplomatic and espionage documents. Finally, the book discusses the alleged Iberian secrecy policies regarding maritime knowledge, explaining why there is no serious reason to consider their success.