This chapter discusses various technologies associated with the design, fabrication, and operation of heat pipes. These include heat transport fluids, containment and wick materials, wick types and characteristics, heat pipe fabrication, environmental influences on heat pipe operation, and heat pipe systems. Almost any fluid may be used in a conventional heat pipe, subject to its ability to wet the wick structure and the containment wall. Virtually all liquids meet the wetting requirement, with the metallic materials usually specified for the heat pipe wick and containment structures. The materials used to fabricate a heat pipe wick and container must be compatible with the selected heat pipe fluid over its anticipated operating temperature range. With the variable conductance heat pipe, the heat pipe temperature can be maintained within a narrow range despite wide variations in conditions affecting the heat input and output rates.