For cooling of electronic or electric equipment, there is a growing industrial demand of high-performance thermal links. One such thermal device is the recently invented pulsating (called also oscillating) heat pipe (PHP). It consists of a closed capillary tube folded into meander and partially filled with a liquid. One side of the meander is in thermal contact with a hot spot, the other with a cold spot. The oscillation of the liquid plugs and vapor bubbles spontaneously occurs after the start of heating by the action of evaporation/condensation at the menisci. The plugs move between hot and cold areas by creating an efficient convective heat exchange. This advantage and also the simplicity of PHP make it highly competitive with respect to other kinds of heat pipes. However, the PHP functioning is non-stationary and depends on a large number of physical and material parameters. As a result, application of empirical correlations is quite unsuccessful, and more sophisticated theoretical and basic experimental studies are necessary. In this chapter, we present the current level of understanding and existing approaches to the PHP modeling and design. We start by describing the basic experiments with the simplest, single-branch PHP that contains only one bubble–plug couple. We show how the results of these experiments help to understand the PHP functioning and introduce the reader to the theoretical and numerical approaches to the PHP modeling by describing the relevant physical phenomena. Finally, we review the state-of-the-art of modeling of the multi-branch PHP. This chapter is complementary to the review of the experimental work on multi-branch PHPs presented in Chapter 1.