Knowledge of the occurrence, storage, and flow of groundwater in mountainous regions is limited by the lack of integrated data from wells, streams, springs, and climate. In his comprehensive treatment of the hydrogeology of the San Luis Valley, Huntley (1979) hypothesized that the underlying, fractured volcanic bedrock of the San Juan Mountains has relatively high bulk permeability and a regional-scale water table with a low hydraulic gradient. Other (some more recent) studies of fractured crystalline bedrock in mountainous terrain indicate that these rock units can act as aquifers (Kahn et al. 2008; Manning and Caine 2007; Robinson 1978; Stober and Bucher 2005). The body of recent work also suggests that the conception that fractured crystalline bedrock is of such low permeability that it constitutes a “no-flow zone” may be inappropriate. In addition to establishing a new baseline, the data presented here are used to test Huntley’s (1979) hypotheses that suggest that the San Juan Mountains may be underlain by a substantial groundwater system. With the advent of computers and digital databases, many types of publicly available data can be used to test hypotheses and provide new insights into mountain hydrogeology at the regional scale in the San Juan Mountains. Plate 16 illustrates processes that suggest several fundamental questions arising from our lack of knowledge of mountain hydrogeology. These questions include: What are the dynamic interrelationships among the tectonics of mountain building, climate, and groundwater, and what are the time scales over which associated processes operate? How does extreme topographic relief allow for groundwater recharge along steep surfaces rather than simply causing precipitation to run off ? How does extreme relief translate into hydraulic gradients that drive groundwater flow? Can extreme gradients drive large volumes of meteoric water deep into the Earth’s upper crust? Once in the subsurface, what are the residence times of these waters? Finally, how does complex geology, commonly associated with mountainous terrain, influence these processes and control potentially heterogeneous and tortuous flow pathways? This chapter presents a synthesis of hydrogeological data, in a reconnaissance style, at the regional scale for the San Juan Mountains. Analyses of these data shed some light on the questions posed earlier for the San Juan Mountains and on mountain hydrogeologic processes in general. These analyses are based on public digital data from geologic and topographic maps, precipitation networks, stream gauges, groundwater wells, and springs. These data can be integrated using the hydrologic cycle expressed as a mass balance between inputs and outputs. The data types noted earlier form the basic set of measurements used to explore, characterize, and quantify elements of the hydrologic cycle. This exploration at a variety of scales yields insight into the relationships among the physical geological framework, climatological and hydrological budgets, and the hydraulic properties of the major aquifers in the San Juan Mountains. Each of these factors has been broken down and investigated separately and then integrated at the end of the chapter, using a conceptual model. Although the San Juan Mountains contain extensive precious- and base-metal deposits that have led to natural and mining-related groundwater contamination, this topic is not addressed here. Interested readers should refer to the extensive body of US Geological Survey work in Gray et al. (1994), Plumlee et al. (1995), Wirt et al. (1999), Johnson and Yager (2006), Johnson et al. (2007), and Church, von Guerard, and Finger (2007). Huntley (1979) also provided a large database for regional hydro-geochemistry of the San Juan Mountains (SJM).