After more than 10 years from its discovery, the accelerated expansion of the Universe has still not found a convincing explanation. The best model so far requires the introduction of dark energy, an hypothetical, enigmatic component that drives the expansion. Building on the first idea of a classical field capable of resisting gravity's pull, Einstein's famous cosmological constant, cosmologists have proposed a number of alternative models. Although none of these has gained decisive support, they have prompted a new era of large scale efforts to map the history and geography of the Universe. In this talk I will review the main concepts concerning dark energy and illustrate the large experimental projects to constrain its properties.