ABSTRACT: Believing that the universe was static, Einstein was forced to introduce a new term into his theory of gravity, the cosmological constant. We now know that the universe is not static, yet the cosmological constant still appears to be a necessary ingredient in order to explain various observations. I will explain what we know about gravity and the history of the universe, and why the cosmological constant poses a deep theoretical challenge for particle physics. I will then describe some of the modern approaches towards a resolution of this mystery.