This paper summarises the results of a study that analysed ways of making South Africa's future energy development more sustainable. The South African economy is comparatively energy-intensive, with total primary energy supply of 11.7 MJ per US$ of GDP on a purchasing power parity basis, compared to 7.9 MJ/$ for Asia and 6.7 MJ/$ for Latin America. Moreover, the high dependence on coal makes the country also very carbon-intensive, with energy related CO2 emissions of 6.7 tonnes per capita, comparable to the OECD average of about 11 tCO2 /cap., and far higher than the non-OECD average of 1.7 tCO2/cap. Important policy initiatives are already under way to im- prove energy efficiency as well as the share of renewable energy. The impact of different energy policies, including alternative technologies for both supply and demand up to 2025, were analysed using the Markal model, a least-cost optimising tool. The ref- erence case is close to the government's Integrated Energy Plan, with CO2 emissions increasing from 337 million tonnes (Mt) in 2001 to 591 Mt in 2025. A cost-effective renewable energy policy scenario would increase the renewable electricity generation from 2,000 GWh in 2001 to almost 18,000 GWh in 2025, with significant contribution from solar thermal and biomass cogeneration technologies. Energy efficiency can make a substantial contribution, especially in industry. The combination of measures would reduce total energy system costs by 16 billion rands ($ 2.2 billion) and CO2 emissions by 770 Mt, each over a 25-year period. The policies analysed here can therefore contribute both to sustainable development and to climate change mitigation.