Experimental data have been obtained for liquid jet impingement cooling of small square heat sources resembling electronic integrated circuit chips. Both free-surface and submerged jet configurations have been studied for a range of velocities, nozzle diameters, and nozzle-to-heater separation distances, with water and a fluorocarbon liquid (3M FC-77) as coolants. Major trends in the data have been explained in terms of the underlying hydrodynamic and thermal phenomena. The data, obtained over parameter ranges applicable to the cooling of microelectronic chips, have been compared with the predictions of previously developed correlations for jet impingement heat transfer and substantial discrepancies between the data and the predictions have been noted. Based on the present data, two new correlating equations, one for free-surface and the other for submerged jet impingement, have been developed and presented.