Hybrid power supply systems integrate the use of different power sources, converters and/or storages for the electrification of maritime applications in order to reduce environmental impacts and contribute to resilience. In the area of port infrastructure, there have been approaches to the hybridisation of RTG cranes, tugs and drayage trucks, which have been equipped with batteries in addition to combustion engines. These approaches can contribute to a reduction in fuel consumption of up to 40% and thus to a reduction in greenhouse gas emissions and air pollutants. With regard to shore-to ship power, there are promising solutions despite the lack of harmonised standards, such as charging with lithium-ion batteries due their high efficiency and power density, as well as battery swapping due to short recharging times and the flexible use of energy from the grid. In the area of hybrid power and propulsion systems, the comparison of diesel engines, fuel cells and batteries shows that diesel engines have lower investment costs, but also lower efficiencies of up to 52% compared to fuel cells with 60%. However, according to the EU HySeasIII project, fuel cells combined with batteries can contribute to a significant reduction in GHG emissions of up to 80%.