This study is based on the archaeological investigation of three wrecks from the Spanish Armada of 1588. As a result of these discoveries it has been possible to assess in practical terms the equipment and resources with which the Spaniards intended to invade England, and to identify their strengths and weaknesses. The ships were in general sturdy and well handled, but most of them were merchantmen and few could stand up well to heavy gunnery. The use of artillery at sea forms a major part of the study, and the extensive collection of guns and associated equipment recovered from the wrecks has helped to show why the Spaniards' performance in this respect was all but ineffective. Relics of the invasion army's weapons and matériel, which include parts of a dismantled heavy siege train, indicate on the other hand that the troops carried by the fleet were well equipped and likely to have been, if given the chance ashore, a formidable and fast-moving force. A fresh examination of the historical material, studied in conjunction with the archaeological evidence, has thrown new light on the campaign as a whole. The threat which England faced in 1588 is shown to have been a very real one. If the original plan put forward by the Spanish commanders to mount a self-contained task force from Lisbon had prevailed, the enterprise would almost certainly have succeeded. But Philip II's insistence on using the Army of Flanders as the main invasion force, with a smaller-scale Armada to escort it across the Channel in barges, gave rise to difficulties which proved insuperable. As an armed convoy the Armada might indeed have proved invincible, but as a battle-fleet it was almost inevitably bound to fail.