According to the International Energy Agency, CO2 emissions have to be cut by a factor 2 if one wants to mitigate the effect on the climate change to an acceptable limit. To reach this target, efficient use and conversion of energy resources is expected to count for more than 50% of the mitigation effort, while renewables and CO2 capture will take an additional 30% . In its recommendation to governments, efficiency is therefore declared by the IEA as being the hidden fuel of the future energy system and will be the 21st century challenge for engineers. At the level of the sustainable energy system design, one could define the challenges as corresponding to the following actions : efficient energy use and reuse to supply energy services and products, efficient energy conversion with CO2 capture, sequestration or valorisation, the proper integration of renewable energy resources and the large scale energy system integration. Process system engineering is an engineering method that supports decision for the process design. In the talk, we will discuss how such techniques can be adapted for the design of sustainable energy systems. We will first discuss how system design methodology has to be adapted to incorporate sustainability indicators by considering life cycle assessment methods. We will then discuss the application of process integration and system engineering techniques to design more sustainable energy systems. We will first analyse the problem of the efficient energy usage and conversion in industrial processes. Examples show that adopting a holistic approach considering heat requirement, energy conversion and waste management can reach very high energy savings. For the process designs, the method is using multi-objective optimisation techniques to generate innovative integrated process designs. Applied to the development of fuel cells systems, innovative designs with electrical efficiencies as high as 82 % can be reached opening new perspectives for cogeneration, CO2 capture and biomass conversion. The design and the integration of biomass conversion processes to produce biofuel , electricity and heat is another example of the application of process system engineering techniques. Synthetic natural gas production with efficiencies as high as 75 % can be obtained. Such plants can also be used as a way to convert power to fuel or as long term renewable electricity storage. The large scale system integration considers the industrial processes and energy conversion unit as decentralised and interconnected combined heat and power plants with connections to smart electrical, heating and cooling district grids. The ultimate system integration considering the use of process system engineering methods to develop industrial ecology concepts will then be presented. In industrial ecology concepts the goal is to reveal synergies between mobility, living, products and waste management services, energy conversion technologies integration and endogenous renewable resources at the level of a community.