Heat pipes are efficient passive devices that can transfer large amounts of heat over long distances with small temperature differences between the heat sources and sinks by evaporation and condensation of the working fluid. Heat can be transferred without the use of any mechanically moving parts such as pumps and active controls in heat pipes. The vapor and liquid circulate in the conventional heat pipes, including thermosiphons, via evaporation/condensation and capillary or gravitational forces. For pulsating heat pipes, liquid slug and vapor plugs in the capillary tube oscillate due to evaporation and condensation. The effective thermal conductivity of a heat pipe can be three orders of magnitude higher than that of a copper rod with the same size. A heat pipe can find its applications in many sectors of industries, including electronics cooling, energy systems, spacecraft thermal control, permafrost cooling, and manufacturing. This book presents current research and development related to the design, applications and technology of various heat pipes, including conventional heat pipes and thermosyphon, pulsating heat pipes, loop heat pipes, and variable conductance heat pipes. Design tools based on computational fluid dynamics simulation and HSHPTM (Heat Sink-Heat Pipe Thermal Module) software are also presented.