Why Is the Universe Accelerating?

Our universe is accelerating, a phenomenon that cannot be accounted for by ordinary matter and conventional gravity. The simplest explanation is to invoke a vacuum energy of 120 orders of magnitude less than the expected amount. Alternatively, there could be a smoothly-distributed, slowly-varying dynamical component, or a breakdown of general relativity on cosmological scales. All of the possibilities are very exciting, and future observations have promise for distinguishing between them. I will give an overview of the theoretical proposals for explaining the acceleration of the universe and the observational constraints which any model must satisfy.