This paper reviews the development, history, theoretical basis, and experimental performance of solid‐state hydrogen detectors under flow‐through conditions available to date such as pyroelectric, piezoelectric, fiber optic, and electrochemical devices. Semiconductor hydrogen detectors will only be reviewed briefly, as excellent reviews on this subject already exist. In view of the fact that almost all the devices that will be discussed later in this paper use Pd as a hydrogen trap, we devote a subsection to examining the role of palladium as a catalyst as well as some of the characteristics of the Pd‐H2 system. Non‐solid‐state hydrogen sensors, such as the flame ionization detector are not the object of this review. A useful feature of this review is a comparison of operating characteristics of each device in a general table in Sec. VII. In that section a general discussion is presented, including a critical comparison of the capabilities and parameters of various solid‐state hydrogen sensors in the form of a table showing data collected from the literature. The Pd‐fiber optic sensor is the most sensitive optical device operating at room temperature. The Pd‐photopyroelectric sensor appears to be most economical and second best in sensitivity at room temperature; it has the best potential for high signal‐to‐noise operation at the widest temperature range, down to cryogenic temperatures. The Pd‐field effect transistor devices exhibit the second highest sensitivity at elevated temperatures.