Recent astronomical measurements have indicated that most of the Universe is comprised of a poorly understood component called dark energy. Very little of its properties are known. Dark matter, another important constituent in the Universe, collapses by gravitational forces and results in the construction of galactic haloes. These haloes eventually merge and construct the largest structures and are called clusters of galaxies. It is now known that the number and distribution of clusters can be used to constrain models of dark energy with high sensitivity as well as teach us about the distribution of dark matter. We describe an ambitious program to use clusters of galaxies to study dark energy over the next several years. We highlight the use of an X-ray cluster survey to use clusters of galaxies for cosmological measurements. We also discuss the use of X-ray spectroscopy and theoretical studies to understand the important physics in cluster formation and evolution. Finally, we describe the emerging observational technique of weak gravitational lensing, which maps dark matter directly. This method will be utilized in great detail with the proposed Large Synoptic Survey Telescope (LSST).