A sustainable energy supply is a precondition for economic and social development and achievement of the Millennium Development Goals. This became clearer than ever in 2002 at the World Summit on Sustainable Development in Johannesburg and at the recent session of the UN Commission on Sustainable Development in New York. In many parts of the world, a large share of the population have no access to modern energy services. At best, towns and industries are supplied with antiquated and unsustainable energy facilities. Worldwide, two billion people with no access to modern energy supplies are forced to use traditional forms of bio-mass such as wood, coal or dung to cover their basic energy needs. This practice is putting enormous pressure on ecosystems and seriously damaging the health of women and children who are exposed to the smoke of traditional fireplaces. According to WHO estimates, some 4,000 people die every day from indoor air pollution. More developed countries are struggling with the ecological consequences of the massive use of fossil fuels. These range from local air pollution to global climate change. The recent rise in oil and gas prices has demonstrated the negative economic and social effects of most countries’ dependency on imported fossil fuels. In light of this situation, there is an urgent need for developing countries to adopt sustainable and forward-looking energy policies. GTZ offers a wide range of capacity-building services to assist in the transition to a sustainable energy future. In this process, GTZ can draw on its long-standing experience with a current 65 energy projects all over the world. Access to modern energy services, the promotion of renewable energy, and greater energy efficiency are the three key elements of any strategy for a sustainable energy supply. The transformation of the global energy system is one of the greatest challenges facing humankind in the 21st century. The contributions to this volume discuss the energy challenges in different parts of the world, illustrate some conceptual approaches to address these challenges, and provide some concrete examples how GTZ, works with partners to find solutions that work.