Gas sensors are widely used in hydrogen technologies. Fuel cell systems, for example, use hydrogen, oxygen, and relative humidity sensors for monitoring the gas streams and providing data for optimising operation of the system and preventing damage to the fuel cell. Hydrogen production systems, e.g., electrolysers, use hydrogen and oxygen sensors for monitoring gas quality online. Systems generating, storing, or using hydrogen also require sensors which are capable of detecting low concentrations of leaked hydrogen. Such hydrogen leak sensors are critical components for safety as they help avoid or mitigate hazardous situations arising from hydrogen combustion or explosion. In this chapter different technologies for detecting hydrogen are described with a short review of their working principle, the history of their development, their deployment, and state of the art. As new markets for hydrogen sensors are rapidly evolving, additional information is provided on novel and emerging hydrogen sensing technologies. Finally a comparison of the performance of available technologies is made and tips on choosing the optimal hydrogen sensor for a particular application are provided. In Chapter 4 other gas sensors, including oxygen and relative humidity sensors, are similarly described.