This paper develops and applies a simple “conditional growth” framework to make long-term GDP projections for the world economy, taking as a starting point recent empirical evidence about the importance of total factor productivity and human capital in explaining current cross-country disparities in GDP per capita levels. Other distinct features of the projection framework include human capital projections by cohorts and implicit allowance for the impact of ageing and potential labour market and pension reforms on future growth in employment levels. In the baseline projection, world GDP would grow in PPP terms by about 3 ¾ % per year on average over the period 2005-2050. When expressed in constant market exchange rates, taking into account future Balassa-Samuelson effects, this projection falls roughly in the middle of the range of long-run scenarios recently developed in the context of greenhouse gas emission projections. The sensitivity of the projection to total factor productivity and population growth assumptions is significant, however, and compounds with deeper sources of uncertainty such as model and parameter uncertainty.