Potential working fluids for heat pipes and loop heat pipes include water, organic fluids, elements, and halides. The paper surveys life tests conducted with 30 different intermediate temperature working fluids, and over 60 different working fluid/envelope combinations. Life tests have been run with three elemental working fluids: sulfur, sulfur-iodine mixtures, and mercury. Other fluids offer benefits over these three liquids in this temperature range. Life tests have been conducted with 19 different organic working fluids. Three sets of organic fluids stand out as good intermediate temperature fluids: (1) Diphenyl, Diphenyl Oxide, and Eutectic Diphenyl/Diphenyl Oxide, (2) Naphthalene, and (3) Toluene. While fluorinating organic compounds are believed to make them more stable, this has not yet been demonstrated during heat pipe life tests. Finally several halides are suitable for temperatures up to 673 K (400°C). Life tests at temperatures up to 400°C were conducted with titanium and three corrosion resistant superalloys, and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide. (Therminol VP-1/Dowtherm A). Ongoing life tests with superalloys/TiCl4 and AlBr3 have been running for 28,000 hours. Ongoing life tests with up to 45,000 hours demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277°C).