The Pulsating Heat Pipe (PHP) is a novel, simply formed, wickless heat pipe that relies on the phase change induced motion of a contained working fluid to transport heat between the evaporator (hot end) and condenser (cold end). The improved heat transfer capability (effective conductivities of more than 18,000 W/m-K at 105 W/cm), simplicity, and reduced mass of PHPs have led many to realize that they are ideal for some spacecraft thermal control applications. Despite the successes in PHP design, considerable work must still be done to improve understanding of PHP operation so that accurate models and design tools can be produced. This paper looks at one crucial, but often overlooked, aspect of PHP operation: the working fluid. In particular, the surface tension effect on PHP sizing is reviewed for terrestrial and microgravity experiments. Then, the primary PHP driving force, the saturation pressure, is discussed with special emphasis on how noncondensable gases affect the degree of sub-cooling. This discussion is followed by a brief discussion of the role that dynamic contact angle hysteresis can play in counteracting the PHP driving force, and a discussion of the superheat and the minimum temperature/heat-flux required for PHP startup. Finally, since this paper clearly shows that traditional heat transfer fluids, such as water, do not work well for moderate temperature PHPs, the authors make their recommendations for PHP working fluid selection.