1967 Outer Space Treaty which requires nations to ensure that activities or experiments of their nation do not interfere with the peaceful exploration and use of outer space. Additionally, under current United Nations law, member states are "prohibited from appropriating the moon."
But what about all those nice moon minerals waiting to be mined? Can anyone appropriate them? A good overview can be found in this e-book: Moon: Prospective Energy and Material Resources.
Find out more about the moon at NASA Moon, including photos, videos, and an interactive map of its