Dam construction has increased exponentially over the past century, primarily in temperate environments. While the impacts of dams in temperate regions have been well-documented, a parallel level of research on dam impacts has not been achieved in tropical environments. The overall objective of this research was to understand the hydrologic impacts of dams in Costa Rica, a representative case study in a tropical environment. To achieve this objective, the following specific objectives were developed: 1) examine the spatial and temporal trend of large dam development within the country; 2) assess large-scale hydrologic impacts (at the national scale); 3) analyze downstream flow of individual dams to determine regional impacts. Analysis of the spatial trend of dam development utilized a geographic information system. The spatial distribution showed no apparent relation to hydroclimate, and additional land-use analysis indicated that basins containing large dams are primarily covered by either forest or crop. Assessment of large-scale impacts used potential reservoir storage to represent the hydrologic impact. Results indicate that large dams in Costa Rica are having a relatively low impact on the surface water component of the hydrologic cycle compared to temperate regions. However, this analysis revealed that two dams, Arenal and Sandillal, are having a disproportionately significant impact on their individual basins. Analysis of flow regime for individual dams followed standard hydrologic analyses of comparing pre- and post-dam discharge data. Variables analyzed included mean, minimum, and peak flows. Results of these analyses revealed that the Arenal- Corobic-Sandillal dam project have resulted in severe disruption to downstream hydrology for all three dams. In contrast, downstream of Ventanas Dam changes in downstream discharge were smaller than those documented for dams in temperate regions. The results of this research indicate that dam impacts in the tropics may be very different from those documented in temperate environments. Consequently, theories developed for temperate areas regarding expected dam impacts may not apply to tropical regions. This has important implications for hydrology, geomorphology and ecology. This study should serve as a step toward development of a more generalized theory of dam impacts in the tropics.