Capillary pumped loop (CPL) and loop heat pipe (LHP), are two-phase heat transport devices which rely on surface tension induced by a fine pore wick to drive a working fluid in a loop. These are based on a working principle similar to that of heat pipes -closed evaporation and condensation cycle being maintained by capillary pumping. CPLs and LHPs are gaining importance as a part of the thermal control system of modern high power spacecraft, electronic thermal management, cryogenics, etc. A mathematical model to simulate the thermo-hydraulic performance of CPLs and LHPs is developed to aid in the design of such a spacecraft thermal control system. In this study a unified mathematical model to estimate thermal and hydraulic performance of a CPL and an LHP -with a two-phase or a hard-filled reservoir is presented. The steady state model is based on conservation of energy and mass in the system. Heat exchanges between the loop and the surroundings and pressure drops in the loop are calculated. The constant conductance regime in a CPL or an LHP occurs when the reservoir is hard-filled. It also occurs in an LHP if the condenser is fully utilised. The heat leak across the wick becomes significant in a hard-filled LHP since the core is no longer saturated and hence the mass flow rate must be calculated using an energy balance on the outer surface of the wick. Theoretical studies indicate that the core of a hard-filled CPL and LHP is always sub-cooled. Hard-filled LHPs (with a bayonet) cannot be operated under all conditions. If the heat exchange between the compensation chamber (of an LHP with bayonet) and the ambient is small then such an LHP will not deprime if the hard-filling occurs before the condenser opens. Deprime due to hard-filling is not expected if it occurs after the condenser opens. A laboratory model is built to demonstrate the operation of these two devices and to correlate the theoretical predictions with the experimental observations. The CPL/LHP laboratory model is fabricated and designed so that different evaporator and reservoir designs can be integrated into the test-rig and tested with different working fluids. Experiments are conducted on a three-port CPL with a tubular axially grooved (TAG) evaporator. This CPL is operated with three different fluids -namely -Ammonia, Acetone and R134a. The CPL is operated for heat loads in the range of 75W to 400W with sink temperatures of -10◦C and 0◦C. The influence of reservoir temperature (35◦C and 43◦C) is also studied. The TAG evaporator is modified to operate in an LHP mode with R134a as the working fluid with heat loads in the range of 75W to 150W. This LHP does not exhibit typical “√” shaped operating characteristic due to large liquid inventory in the compensation chamber (CC). The R134a based LHP results suggest that large liquid inventory (in the CC) and absence of secondary wick significantly influence the thermal coupling between the core and the compensation chamber. Experiments are also conducted with a flat plate (FP) evaporator, in LHP operating mode, with Ammonia as the working fluid. This LHP can transport heat loads from 25W to 300W with a sink temperature at -15◦C. The experimental results indicate that ammonia is the best working fluid (moderate temperature regime) among all the working fluids tested, and that evaporation heat transfer coefficients in sintered Ni-wick are better. The correlation of the predicted temperatures on the transport lines and the saturation temperature (in LHPs) with the observations is good. Some of the salient conclusions from these experiments are that mass of charge can significantly influence the operating characteristics of a TAG LHP, even though the fluid in the CC is in two-phase condition. Theoretical predictions can be significantly affected when thermal and hydraulic development lengths in the condenser are comparable with the length of the sub-cooling section.