100 years ago, Einstein completed the formulation of his General Theory of Relativity, which has proved to be spectacularly successful as a classical theory of gravity. Since then, enormous advances have been made in unifying all of the other fundamental forces in nature, but the problem of completely unifying gravity and particle physics remains as one of the major challenges in theoretical physics today. In this talk, the principles of general relativity will be described, and why it is so difficult to extend it to a consistent quantum theory that could unify it with the other fundamental forces. String theory currently seems to provide the only promising avenue for such a unification. There have already been some significant successes. I shall also describe some of my recent work on building a braneworld model of our four-dimensional universe within the framework of string theory.