This study indicates that improvements in the efficiency of energy supply and end-use technologies can be both environmentally beneficial and potentially financially rewarding. A total discounted (at a 6% real annual rate) incremental capital investment of US$1.1 to $3.6 trillion (1990 dollars) would be required to improve energy efficiency sufficiently for a reduction of global CO2 emissions in 2050 by one-third. However, this investment would ultimately be offset by savings in the discounted energy cost, which could total US$4.8 to $7.1 trillion for the same period.