This article is about how science provides the answer to the apparently simple question “What is the temperature?”. The question is asked every day in every field of science and engineering. We consider first the purpose of temperature measurement, and compare and contrast temperature metrology with metrology for mass and length. We discuss current practice—which is a description of the International Temperature Scale of 1990 (ITS 90)—and then look at the rationale for a possible re-definition of the unit of temperature, and the role of measurements of the Boltzmann constant in that process. We then review possible techniques for measurement of the Boltzmann constant, and finally reflect on a recent development in experiments to determine the Boltzmann constant using accurate measurements of the speed of sound in argon.