This is an experimental and numerical study of natural convection and forced convection air cooling of plate finned heat sinks. The study documents quantitatively two effects : (1) the orientation of the fin array relative to the gravitational field in natural convection, and relative to a free stream in forced convection, and (2) the tilting of the crests of the plate fins relative to the approaching flow. In each configuration, the results based on complete three-dimensional numerical simulations of the flow and heat transfer confirm the validity of the results determined based on direct measurements. For example, it is shown that the overall thermal conductance of the heat sink increases when the fin crests are inclined so that they face the approaching fluid. This augmentation effect increases as the coolant velocity increases. The thermal performance of arrays positioned perpendicularly to the how is also documented.