Experiments were performed to characterize heat transfer to a normally impinging air jet from surfaces modified with arrays of fin-type extensions. Heat transfer enhancement for six fin geometries was evaluated by comparison with results for a smooth, flat surface. Average Nusseh numbers (Nu) and overall system effectiveness (r.) are reported as functions of fin type, jet Reynolds number {Re), and nozzle-toplate spacing (z/d) for two nozzle diameters. Enhancement of the absolute rate of heat transfer, as compared to the smooth surface, was demonstrated by a factor ranging from 1.5 to 4.5. The system effectiveness as a function of Re exhibited strong fin type dependence due to signil]cant variations in the total surface area and average Nusselt number. The fin type dependence of Nu as a function of Re was found to be a result of variations in the turbulence level, fluid velocity, and the percentages of total surface area exposed to normal, oblique, and parallel flow. The average Nusselt number correlated well in the form Nu = .4 Re". For the modified surfaces, the system effectiveness decreased monotonically with increasing _-/d in contrast to the smooth surface behavior.