This thesis studies social movements and their interaction with legal institutions, particularly constitutional courts, in their pursuit to influence abortion law reform or counter-reform in Latin America. More generally, it intends to contribute to the study of the conditions and ways in which movements in civil society may influence legal change. It introduces an analytical framework that combines three theoretical perspectives developed in separate fields of scholarship, which are usually not connected: social movement theory, democratic constitutionalism and legal mobilization studies. The underlying premise, following democratic constitutionalism, is that social movements can be central actors in the generation of a discourse that begins from below and that may influence the law officially sanctioned by the state. The cases in this study - Colombia, Mexico, Brazil and Argentina - show that recent changes to the abortion laws in Latin America have responded to direct claims by feminist actors in civil society. Over the last decade, constitutional courts have sided for the first time in the region with feminists' claims to decriminalize abortion in certain circumstances, and their decisions have been in line with the human rights discourse and particular ways of framing the abortion issue advanced by feminists to ground their rights claims in this field. An analysis of the interaction between social movements and the legal system in each case is carried out through an analytical narrative, drawing on original semi-structured interviews conducted from 2010 to 2013 with social movement activists, lawyers, health professionals and academics in each country, as well as on primary source documents and secondary sources, mostly produced by Latin American feminists. The main case law by constitutional courts in each case is analyzed with attention to the socio-legal process developed around judicial decision-making, and the relationship between courts and social movements.