Spacepower, analogous to airpower and sea power, is the ability to use the space medium to project military power. Since the end of the Cold War, the contributions of spacepower to national security and military operations have become increasingly visible in the open press, leading to an increased stated need for a comprehensive spacepower theory. This research is focused toward that need. It explores the central themes and specific points of the "theoretical masters" of land, sea and air, in order to draw analogies to the emerging presence of military spacepower. Space will continue to aid combat in other mediums as it offers persistence, range, and near instantaneous speed-enabling a global presence that is unmatched by capabilities in any other medium. However, for spacepower to emerge as a fully competent component of the future joint force, or simply to mature as a combat arm within the US Air Force, a body of serious thought must be given to why we need spacepower and what we intend it to do. Through this analysis, five lessons emerge for spacepower theory: 1) Spacepower must control space lines of communication; 2) Decisive points are key to space control; 3) Spacepower requires superior observation capabilities and the ability to take offensive action; 4) Robust spacepower could lead to enemy paralysis; and 5) Spacepower requires masters of the space medium.