A variety of observations have led cosmologists to conclude that the universe is dominated by a mysterious form of "dark energy" (in addition to the well-established "dark matter", which now seems prosaic by comparison). This dark energy could be vacuum energy (a cosmological constant), or something dynamical and slowly evolving. 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 dark energy and the observational constraints which any model must satisfy.