Hydrogen is a unique fuel in many ways. It has the potential to eliminate both carbon monoxide and carbon dioxide emissions. However, it could increase NOx emissions compared to more conventional fuels. Hydrogen has many unique properties such as a wider flammability range, higher adiabatic flame temperature, higher flame speed, lower heating value on a volume basis, and lower combustion air requirements. Some potential safety concerns include a higher propensity to flash back on pre-mix burners because of the higher flame speed and to leak because H2 is a very small molecule. Because of its many unique properties, there are some important design considerations when retrofitting a combustion system designed to use a conventional fuel like natural gas to use hydrogen. This paper investigates some of the key differences when using hydrogen as a fuel compared to typical hydrocarbons. It considers the impact on: the fuel and air delivery systems, flame detection, burner design, heat transfer, and pollution emissions. It includes two specific examples of implementation in process heaters and a boiler.