The definition of alternative fuels is evolving from a material-based approach (focusing on what is used) to a process-based approach (emphasizing how fuels are produced). For alternative fuels to be considered suitable for environmentally friendly ships in future, it seems that they need to meet three following criteria. First, to truly estimate the impact on climate change, carbon dioxide (CO2) equivalent greenhouse gas (GHG) emissions considering global warming potential should be evaluated, rather than just CO2 emissions from fuel use. Second, well-to-wake GHG emissions should be evaluated considering lifecycle assessment from the production to use of a fuel. Third, specific quantitative criteria will be required to qualify a fuel as an alternative fuel, ensuring meaningful reductions in GHG emissions. The recently announced 2023 IMO GHG strategy, along with discussions at the Marine Environment Protection Committee shows these features well. This book introduces alternative fuels for environmentally friendly ships including the topics in liquefied natural gas, hydrogen, ammonia, biofuels, e-fuels and carbon capture, utilization, and storage.