Development of state of the art electronics forces thermal engineers to develop more efficient and innovative heat transfer devices. Modern electronics do not only dissipate more heat per area but also their operation is subject to temperature stability. Loop Heat Pipes offer an important advantage over other heat transfer devices with their unique characteristics. These self-regulated devices utilize the latent heat of the working fluid circulating between a hot source and cold sink by using thermodynamic principles and capillary forces. Experimental work is performed, on two different LHPs under ambient conditions, to investigate the operational characteristics of LHPs and to collect data under different operating conditions for the validation of the numerical model developed in this research. The main investigated characteristics comprise of the operating temperature and its oscillations at given operating conditions, the LHP response to a change in the operating conditions, and the location of the two-phase/liquid interface inside the LHP condenser. Furthermore, the empirical correlations, required to calculate the LHP heat transfer coefficients and pressure drops, are chosen and verified for the numerical model. Following the correlation verification, a modular numerical model is developed and validated to predict the steady-state and transient operation of an LHP with minimum accommodation parameters. The sensitivity studies of the modelling parameters are conducted to investigate their effects on LHP operation. The model can be easily modified or improved because of its modularity feature. Additionally, the minimum number of the accommodation parameters allows the model to predict LHP behavior by relying on only one set of power cycling test results. The mathematical model can be used not only as a design tool in the research and development of new LHPs but also to examine and troubleshoot the operation of an existing LHP in remote locations such as onboard a satellite orbiting Earth.