During the twentieth century, the science of hydrogeology focused on establishing and refi ning fundamental principles and developing tools to study groundwater flow, well hydraulics, hydrogeochemistry, and contaminant hydrogeology. By the end of the century, the science evolved to assimilate principles and expertise from other disciplines, including surface water hydrology, chemistry, microbiology, geophysics, and ecology. In this chapter, we review seminal achievements in hydrogeology from 1963 to 2013, focusing on work by recipients of the Hydrogeology Division's O.E. Meinzer Award, one of the most prestigious and coveted awards in hydrogeology. The canon of 116 Meinzer Award papers, reports, and books refl ects the trends in hydrogeological research since the early 1960s. We also discuss other contributory papers by Meinzer awardees and related work by other scientists, and cover some research areas that have not been recognized by the Meinzer Award. We anticipate that the contributions of future Meinzer awardees will continue to document leadership in hydrogeology, perhaps in areas that have not yet been recognized by the award, including hydrogeoecology and hyporheic processes, submarine groundwater discharge, multilevel slug tests and hydraulic tomography, heat as a groundwater tracer, hydrogeophysics including remote sensing, and regional groundwater hydrology applied to issues of climate change.