The Chilean national learning outcome assessment system (Sistema de Medicion de Calidad de la Educacion, SIMCE) has carried out census-based assessments since 1988 and publishes the results at both the national and school levels. During its 20 years of existence, SIMCE has experienced changes in its institutional framework, objectives, methodologies, uses, public perception and legitimacy. After two decades, SIMCE faces new challenges from a society that is pushing to increase accountability and consequences attached to assessment outcomes. The first section of this paper describes SIMCE's main features, purposes, institutional framework, and strategies for communicating results. Section two reviews SIMCE's history from limited to broader legitimacy, and from low to high stakes. In the Latin American context, the relevance of a legal and institutional framework guaranteeing the continuity and sustainability of a national assessment program is highlighted. The third section analyses the uses of the information SIMCE generates. The authors stress that the main lessons learned are that a census-based assessment system focuses attention on learning outcomes and impacts everyone, that published results make all actors accountable, that effectively communicating results is as important as accurately generating them, and that these elements help develop a culture of evaluation. (Contains 2 tables and 5 notes.)