In 1994 Colombia started replacing its state-run and pay-as-you-go (PAYG) pension system by a privately-run and fully-funded scheme. This study analyzes prospective fiscal and macroeconomic implications of this reform. It compares the features of Colombia's old and new pension system, puts them into broader international context, and looks at the reform transition. Numerical simulations for the government's reform transition reveal implicit PAYG debt levels and corresponding reform transition deficits that are high relative to other countries, considering that Colombia's old pension system was characterized by low coverage, low system maturity, and a young population. Simulation results show that output could increase by 10 percent due to higher future saving caused by financing the pension deficit by a fiscal contraction - but this would occur only in the very long term. Sooner and possibly larger gains could be reaped from higher employment and production in formal sectors, and the development of capital markets spurred by the reform. In addition, Colombia's new pension system - which includes a redistributive pillar targeted at the poor -- is potentially more equitable than the old scheme. To reap these efficiency and equity benefits, however, the Colombian government would have to adopt complementary reforms. They include giving the private fully-funded pension pillar a commanding role, supporting the development of capital markets, and bolstering formal-sector employment.