High-temperature processes in the field of, e.g., energy conversion or chemical technologies require sophisticated process monitoring and control to ensure high-efficiency, low pollutant emissions, and safe operation. These objectives can only be achieved by in-situ control of the processes. The increasing combustion of biofuels, organic waste, wood, etc., tightens the demand for process control even more. Properties to be monitored include temperature, gas composition, pressure, torque, mechanical integrity, and state of functional components. In this chapter, an overview about current gas sensor principles for operation temperatures above 500°C is given. Thereby, the related range of measurement, the selectivity, the sensitivity, the response time, and the long-term stability are presented along with application examples. Since the selection of sensor materials plays a crucial role at high temperatures, material aspects are an essential part of the chapter. The discussion of solid-state sensor principles includes potentiometric, amperometric, resistive, and resonant sensors.