Revolutionary technological innovations over the past half-century, many of which evolved from defence systems, have transformed our relationship with the seas. The range of activities - scientific, commercial and military - that are now conducted on, below and above the waters has greatly increased, as has our scientific knowledge. This work provides a review of both the civil and the military aspects of seapower as we move into the twenty-first century. In the wake of the Gulf, Bosnian and Kosovan conflicts, not to mention numerous sea-based humanitarian interventions all around the world, there is huge debate about the function and composition of navies. Can we now take western sea-control for granted? How should we think of the projection of military power ashore in the age of cruise missiles? Can medium powers like Britain realistically continue to aspire to the possession and employment of modern and effective seapower, especially in the shape of amphibious forces and modern aircraft carriers? The conclusion of eminent naval experts is not only that we can, but that we must. This applies not only to Britain, but also to her allies in Europe, to the United States and to the burgeoning seapowers of the Pacific Ocean.