A discussion of many of the air-flow instruments currently used in compressor research is given. A history of the development of some of the instruments is presented together with the reasons for adopting the present designs. Optimum dimensions, based on test data, are included for claw probes, cylindrical probes, spherical probes, and averaging probes. Errors encountered in temperature measurement by means of thermocouples are reviewed, and methods for reducing these errors are suggested. A discussion of traversers is also included.