The achievement gap in many developing countries is defined in terms of rich/poor and public/private. The prevailing explanation for the "developing" achievement gap is an underfunded, inefficient, and/or inadequately supplied public school sector. Via an analysis of a Colombian voucher experiment, this article examines the extent to which income-contingent vouchers can narrow the achievement gap and provide a cost-effective method for increasing secondary school enrollments. Despite structural and implementation flaws, which diminish the program's impact on achievement and enrollments, its successes strengthen the argument that implementing an income-contingent voucher program can help narrow the achievement gap in developing countries. To cost-effectively increase enrollments, however, significant modifications and expansions to the program would be necessary--as explained in the conclusion of this article.