The Issues Remaining in Low Temperature Physics and Thermometry

I will motivate this topic by discussing the two limits of temperature: the highest conceivable temperature as well as the very lowest temperature attained in a physics laboratory. I will then review the development of temperature scales, touching briefly on the biographies of the two most important contributors: William Thompson (later, Lord Kelvin) and Daniel Fahrenheit. I will then discuss the present issue in thermometry: The current international temperature scale, ITS-90, does not extend below 0.65 K. Many low-temperature experiments have been conducted far below this limit and call for an extension of this scale to a temperature at least as low as 0.001 K. The status of the international effort to achieve this goal is reviewed.