The variation in heat transfer between a fine wire and the surrounding fluid has traditionally been the most accepted method for measuring the detailed movements of a flow fluid. This paper reviews the present understanding of thermal techniques as used in research and recent advancements of the technique. While the basic research sensor is still a fine wire, modern electronics has greatly simplified data analysis as well as established constant temperature operation of the sensor as the control method of choice. Film sensors have extended the use of thermal sensors to new applications. Comparisons with laser Doppler velocimeters help to identify experiments where thermal sensors still represent the best choice. The ability of modern electronics to easily address the typical nonlinear response of thermal sensors has resulted in increased use for commercial applications. While in these applications fast response can be a factor (e.g., fuel flow control in engines), generally only average flows are needed. As a result, calibration shifts due to deposits on the sensor surface when exposed to contaminated fluids can generally be reduced to acceptable levels by using larger sensors.