Reducing fuel consumption on board of ships is today a priority for the operators in view of the increasing prices of fuel and of the pressure by the international regulatory. Furthermore, at least in the case of CO2 emissions, there is a virtuous relationship behind environmental friendliness and economic benefit, as reducing marine fuel consumption by increasing the overall energetic efficiency of ships automatically means cutting down the emission of pollutants in the air. Finally, it must be reminded that Energy Efficiency for complex ships such as cruise vessels requires a holistic approach as all the main ship systems and processes related to the generation / transformation / consumption of energy must be in principle addressed and considered in an integrated manner if one aims to achieve an overall improvement. To this regard, particular attention should be paid to the process of Waste Heat Recovery, considering also the growing diffusion of low-grade heat recovery units such as Organic Rankine Cycle and Absorption/Adsorption Chillers units.. On the other hand, given the aforementioned complexity of large modern passenger ships, an optimized systematic Energy Management policy strongly needs a dedicated Decision Support System. To avoid unnecessary proliferation of functions / systems, it is a natural choice that Energy Management onboard shall be performed by the Integrated Automation System. In order for the Automation System to evolve towards a support system for operational decisions and further towards a closed-loop control of the energy processes / plants on-board, it is necessary to implement in the system more knowledge on the plants / processes involved in the different tasks.