Energy demand has been systematically increasing, and the trends indicate that this pattern will not change in the next decade. The use of fossil fuels is responsible for the emission of gases related to global warming together with air pollutants such as toxic species and particulate matter. The realization of a green economy requires the safe use of clean and virtually inexhaustible energy sources. In that sense, hydrogen obtained via solar water splitting has potential for becoming an alternative fuel. The safe production, transport, and use of hydrogen as a fuel source require the improvement of currently existing hydrogen sensors. This chapter critically reviews the current technologies and the main research trends in hydrogen sensing. The demand on energy is systematically increasing, and the trends indicate that this demand will keep increasing in the years to come. Nowadays, fossil fuels such as petroleum and natural gas are the main energy sources. However, these are nonrenewable and responsible for the emissions of both greenhouse gases (e.g., carbon dioxide), leading to global warming, and air pollutants (e.g., nitrous, sulfur compounds, and particulate matter), causing respiratory and cardiovascular diseases. The World Health Organization estimates that air pollution is responsible for about 5.4% of all deaths, worldwide, namely, some seven million deaths yearly (World Health Organization. Mortality and burden of disease from ambient air pollution. http://www.who.int/gho/phe/outdoor_air_pollution/burden/en/ . Accessed 14 Dec 2025). Therefore, it is not surprising that the last years have seen important efforts for identifying and developing new energy sources. Renewable sources of energy such as solar, wind, geothermal, hydroelectric, or tidal energy present some limitations in capturing, storing, and transporting this energy. Batteries are in constant evolution for ameliorating performance; however, the range of standard vehicles still compares favorably to the one achieved by electric vehicles and the production of such batteries also has an impact in the environment. An ideal fuel should be energy-efficient; clean; safe to produce, store, and use; and virtually inexhaustible. The fact that hydrogen possesses most of these qualities explains why it is being evaluated as a serious option for the replacement of fossil fuels and natural gas in industrial and transport applications (Winter, Int J Hydrog Energy 31:1623–1631, 2006). Hydrogen is a very efficient energy carrier, namely, its energy density in MJ/kg nearly triples the one of natural gas, gasoline, diesel, or LPGs. In addition, its combustion (oxidation) produces water and results in a virtually zero emission fuel (Frolov et al., Int J Hydrog Energy 38:4177–4184, 2013). Pure hydrogen has been manufactured for more than a century and used safely in many petrochemical applications and in rocket propulsion (Gupta, Hydrogen fuel-production, transport, and storage, 1st edn. Taylor and Francis Group, New York/Boca Raton, 2008). However, a few severe accidents with important impact have prevented hydrogen to become largely accepted as other fuels in vehicles or for in-home use (Mirza et al., Int J Hydrog Energy 36:2068–2077, 2011). Hydrogen is 14 times lighter than air, so it diffuses swiftly. Its absorption on metals causes a loss in ductility (embrittlement), which may eventually result in the failure of the container employed for storage, transport, or delivery. A hydrogen leak may become a serious incident, since hydrogen shows a very wide range of flammability (from 4% to 75% in air), very low ignition energy, and high flame velocity. In Najjar (Int J Hydrog Energy 38:10716–10728, 2013), a valuable discussion on the risks associated to hydrogen and the safety measures to be implemented is given. Therefore, the realization of a hydrogen economy demands high degrees of safety to be achieved, and in particular, this demands the development of reliable, inexpensive, highly responsive, and selective hydrogen sensors. Such sensors should be able to detect and raise an early warning in case of a hydrogen leak. Hydrogen sensors would be of use along the whole chain of hydrogen production, storage, transport, and end-point application. According to the US Department of Energy, hydrogen safety sensors research and development activities should target at meeting the specifications as follows (US Department of Energy. R&D targets for hydrogen safety sensors. http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/safety.pdf . Accessed 14 Dec 2025): Here, we review an updated state of the art for the detection of hydrogen employing gas sensors, discuss strengths and weaknesses of the different options available, and identify the most promising approaches. In addition, recent developments of nanomaterials with potential for the realization of inexpensive and highly sensitive hydrogen sensors are presented and critically discussed.