This study investigates opportunities and constraints in relation to the operation of marine Diesel engines in a dual fuel mode with diesel and hydrogen. Increasing costs of fuel place an additional burden on the marine industry and in some sectors, such as the fishing industry, the long term viability of ongoing operations at high fuel costs is no longer given. Indications are that it is possible to produce hydrogen at competitive costs from renewable energy sources, and approaches of how hydrogen can be used to reduce the fossil fuel requirements in the marine sector are identified. Findings presented in this book are the results of a research activity based at Lews Castle College, UHI, on the Isle of Lewis, Outer Hebrides of Scotland.