In its new energy and climate change policies, released in January 2008, the European Union has made the most daring change of energy policy up to date. It has decided to set up a binding target of 20% renewables in final energy demand, to reduce greenhouse gases emissions by 20% and to reduce the primary energy use by 20%, by 2020. A significant technology shift will be necessary to fulfill such goals, but it is already clear that technologies that will be used are available, mainly in the area of renewable electricity and heat generation, biofuels, energy efficiency, especially in buildings and transportation, cogeneration, nuclear energy in those countries in which it is politically acceptable, which will all be achieved by a combination of binding targets, certificate trading and fiscal policies. With a change of direction also anticipated in the United States, the intertwining of security of energy supply and environmental protection is becoming a new global tenet. The 4th Dubrovnik Conference on Sustainable Development of Energy, Water and Environment Systems was dedicated to the improvement and dissemination of knowledge on methods, policies and technologies for increasing the sustainability of development, taking into account its economic, environmental and social pillars, as well as methods for assessing and measuring sustainability of development, regarding energy, transport, water and environment systems and their many combinations. Sustainability being also a perfect field for interdisciplinary and multi-cultural evaluation of complex system, the Dubrovnik Conference has during the first decade of the 21st century become a significant venue for researchers in those areas to meet, and discuss, share, and disseminate new ideas.