The growing maritime industries will be decarbonized by replacing traditional fossil fuel power systems with renewable energy systems. Batteries dominate automobile applications through flexible operation, large-scale production, and renewable energy storage. However, low volumetric and gravimetric power densities make marine battery systems large and heavy. Renewable fuels with fuel cells, batteries, and storage components will be integrated into hybrid power systems. Such systems are light and compact, suitable for ship applications. Solid oxide fuel cells (SOFC) and proton-exchange membrane fuel cells (PEMFC) are promising fuel cells for generating electricity from renewable fuels. This chapter will introduce the ship's hybrid energy systems in terms of technical details, demonstrations, and market potential.