Heat pipes are promising heat removal devices widely used in a variety of elds ranging from thermal management of electronic components to terrestrial and aerospace applications. Their working principle, phase change of a working uid, makes them superior to other conventional cooling methods. This thesis study focuses on at grooved heat pipes and the e ects of working uid, lling ratio, groove density, and input heat ux on their thermal performance are investigated. During the study, two aluminum heat pipe generations and one silicon heat pipe con guration, each having a set of di erent groove densities, are fabricated. In each set, di erent methods of heating and cooling are applied. In all the experiments on aluminum heat pipes, the working uid is isopropyl alcohol due to its wetting characteristics that makes it compatible with the aluminum surface. For the case of silicon, the heat pipes are charged with isopropyl alcohol and water. The optimum lling ratio, corresponding to the minimum temperature di erence along the heat pipe and maximum e ectiveness, is reported for each heat pipe. Moreover, as one of the operational limitations of heat pipes, the occurrence of dryout is visually observed and its extent is reported for each heat pipe operating at di erent lling ratios under di erent heat inputs. Furthermore, to nd the heat input to the heat pipes of rst generation and to simulate the phase change in one of the heat pipes of second generation, two 3-D computational models are developed and temperature distribution along the heat pipes are veri ed by the experimental results.