To set energy RD&D priorities, information is required on the longterm prospects of existing, improved and new energy technologies. The Netherlands Agency for Energy and the Environment (Novem) commissioned a programme, called SYRENE, to address this issue. The programme comprised analyses on different levels, including the characterisation of energy technologies and the assessment of the prospects of energy technologies on different system levels. Conclusions are drawn on the relative long-term prospects of energy technologies with focus on reduction of CO2 emissions.