The Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, Including the Moon and Other Celestial Bodies (commonly referred to as the “Outer Space Treaty”) have been successfully regulating the space activities of states for more than half a century. The treaty is a tremendous achievement of the international community reached in the challenging and complicated “Cold War” era. It sets up the principles of international space law such as the use of outer space for peaceful purposes and for the benefit of all countries, the prohibition of national appropriation of outer space and celestial bodies, the liability of states for damages, caused by their space objects, the special importance of astronauts as “envoys of mankind” etc. These timeless principles govern and must continue to govern space activities of states in the future. Their importance and relevance cannot be affected by the development of technology. However, some aspects of the Outer Space Treaty are subject to heated discussion and even concern not just in the academic circles, but also on governmental level. The lack of definition of outer space and article IV of the Outer Space Treaty are often recalled as the main causes of concern. This article will discuss these and other aspects of the Outer Space Treaty in their relationship with the prevention of militarization of outer space.