The compression ignition (CI) engine has been the workhorse of the global economy for several decades. However, owing to the need to combat global climate change, the long-term survival of the CI engine depends on its ability to operate with alternative carbon-neutral fuels. Hydrogen has excellent combustion characteristics, making it suitable for application in CI engines. However, it has a high auto-ignition temperature, which limits its operation as a secondary fuel in a dual-fuel engine, unless an ignition source, such as a glow plug, is present. Hydrogen dual-fuel CI engines exhibit diesel-like combustion characteristics and are less prone to the loss of combustion control at high loads or the occurrence of abnormal combustion events, which often occur in hydrogen spark-ignition engines. The biggest challenge associated with the implementation of hydrogen dual-fuel engines is the high combustion temperature, which results in the production of high nitrogen oxide (NOx) emissions that must be addressed using aftertreatment technologies. Nevertheless, hydrogen dual-fuel CI engines are a low-cost and mature technology that can significantly reduce carbon emissions. Over the past few decades, significant research has been conducted on the adaptation of this technology as a medium-term decarbonization solution.