Submillimeter galaxies (SMGs) are thought to represent the link between high-redshift star-forming galaxies and passive early type galaxies, with the huge merger driven bursts consuming up most of the available gas and driving the morphological transition to a spheroid. These hyper-luminous high-z galaxies commonly trace regions of high galaxy overdensity, and may be directly related to the formation of galaxy clusters and their giant central ellipticals. Molecular gas plays a central role in our understanding of the nature of these often heavily obscured systems. It represents the material that stars form out of, and its mass, distribution, excitation, and dynamics provide crucial insight into the physical processes that support the ongoing star formation and stellar mass buildup. I will discuss the most recent progress in studies of gas-rich galaxies at high redshift through large submillimeter surveys with the Herschel Space Observatory and detailed follow-up studies of the molecular gas properties of SMGs with CARMA, the Jansky Very Large Array (JVLA), the Plateau de Bure interferometer, and the Atacama Large (sub)Millimeter Array (ALMA).