The heat transfer characteristics of highly populated pin-fin arrays have been investigated for three different orientations in the gravity field: (1) horizontal fins and vertical baseplate, (2) vertical fins and horizontal downfacing baseplate, and (3) vertical fins and horizontal upfacing baseplate. Experiments were performed in air to measure the combined natural convection and radiation heat transfer, and the radiation was determined analytically. Parametric variations were also made of the number of fins and of the baseplate-to-ambient temperature difference. In general, among the three orientations, the vertical upfacing fin array yielded the highest heat transfer rates, followed by the horizontal fin array and the vertical downfacing fin array. With an increase in the number of fins for fixed values of the other parameters, the heat transfer rate increased at first, attained a maximum, and then decreased, thereby defining an optimal fin population. The fractional contributions of radiation to the combined-mode heat transfer were generally in the 25-40% range, with the larger contributions occurring at the smaller baseplate-to-ambient temperature differences. Comparison of the pin-fin results with those for plate fins tends to encourage the use of pin fins.