Carbonate aquifers are some of most challenging to characterize because dissolution can greatly enhance permeability, but its effects are often difficult to determine. This study analyzes data from caves, wells, and tracer tests to explore the extent of solution channel networks and the factors that influence their development. The nonlinear dissolution kinetics of calcite, mixing of waters with different CO2 concentrations, and unstable dissolution fronts all promote the development of solution channels, which are widespread in unconfined carbonate aquifers. Fractures are important for guiding channels at a local scale, but hydraulic gradients are the dominant control at a regional scale. Channels provide continuous, large-aperture pathways that result in rapid groundwater flow. Small channels are much more abundant than large channels, and often account for most of the permeability measured in wells. Caves represent the largest channels; they are more common in limestone than in dolostone, and the development of caves rather than smaller channels is also favored where there is sparse fracturing, low matrix porosity, and the presence of sinking stream recharge rather than percolation recharge. Solution channel networks have fractal properties, and their presence explains why carbonate aquifers have higher permeability than aquifers in any other rock type.