Technological innovation is fundamental for rendering the energy economy cleaner and more efficient with concomitant economic, developmental, and environmental benefits. This paper discusses aspects of R&D and 'learning-by-doing,' the main contributors to technological change that are complementary yet inter-linked. The relationship between the level of national energy R&D investments and changes in the trajectory of the country's energy system is complex; targeted efforts to promote deployment of new energy technologies play a major role in translating the results of R&D activities to changes in the energy system. Learning-by-doing is an important element of deployment, but it remains largely poorly understood. Hence this phenomenon needs to be,unpacked' and its various aspects analyzed in detail, so as to allow better design of early deployment efforts to enhance learning gains. This paper highlights how public R&D and deployment efforts must work in tandem to expand the portfolio, and realize the potential, of new and improved energy technologies. (c) 2005 Elsevier Ltd. All rights reserved.