Recently available results from the 1973 census indicate Colombia is moving through a fertility transition as rapid, or very nearly as rapid, as any on record. Estimates of fertility at the national level and for subsections of the population are presented and compared with evidence from earlier fertility studies. A review of the demographic, economic, and social contexts in which thedecline took place indicates that Colombia's society was highly, if unevenly, modernized before fertility began to fall. Organized family planning programs are seen to have played a facilitating role, responding to a demand for fertility limitation that was generated by general socioeconomic development.