An experimental study of augmented heat transfer in horizontal tubes under laminar flow conditions has been carried out. Small-scale constant heat flux tests were conducted with two twisted-tape inserts, one internally finned tube, and two static-mixer assemblies. Isothermal pressure drops and local heat transfer coefficients were measured with water and ethylene glycol as test fluids. Large-scale constant wall temperature tests were carried out using a plain tube, a twisted-tape insert, and an internally finned tube with spiral fins. Isothermal and diabatic pressure drops and average heat transfer coefficients, for both heating and cooling, were measured using ethylene glycol as the test fluid. Increases in pressure drop−especially for the static-mixer assemblies−were observed, along with increases in the heat transfer, for all seven augmented tubes tested. Based on a constant pumping power performance analysis, the internally finned tube used in the large-scale tests looks especially promising, for heating as well as for cooling.