Instrumentation based on acoustic principles is finding widespread use in utility boiler applications. This paper describes how acoustic systems are currently being used to measure volumetric flow rate in large ducts and stacks for continuous emission monitoring system (CEMS) applications as well as for the measurement of high-temperature flue gases and boiler control. Acoustic pyrometry is a useful tool for the measurement of gas temperatures in the furnace and superheater regions in boilers. Results have shown that acoustic pyrometry is easy to apply, accurate, and noninvasive. Gas temperature measurements are provided and displayed on a continuous real-time basis for single path volumes in various furnace regions and for multiple path arrays in the furnace exit gas plane. The determination and presentation of isothermal maps of gas temperature provide a new tool for operation and maintenance diagnostics, not achievable by any other currently known method. Acoustic pyrometers are now being used to help identify and correct burner problems, slagging problems, and furnace overheating before these conditions can adversely affect operations.