The thesis analyses the use of external sources as a tool for the evolutive interpretation of the American Convention on Human Rights, in light of the practice of the Inter-American Court of Human Rights and the general rule of treaty interpretation. By external sources, this study refers to instruments adopted outside of the scope of the Inter-American System of Human Rights, ranging from universal and regional conventions to soft law instruments. Examples of external sources that have guided the Court’s evolutive interpretation of the American Convention and have accordingly contributed to an expansion of the content and scope of the provisions of the Convention include the Convention on the Rights of the Child, rules of international humanitarian law and resolutions of international organizations. This thesis develops around the research question on whether the broad use of external sources in the Inter-American System of Human Rights constitutes an interpretative method based on the Vienna Convention on the Law of Treaties of 1969. This research question encompasses two sub-questions, namely: (i) whether a certain level of consensus among the states parties to the American Convention should be a pre-condition for the use of an external source during the interpretation of this treaty; and (ii) whether soft law instruments should be used as an interpretative tool for the evolutive interpretation of the American Convention. To answer these questions, the thesis engages in an analytical and critical investigation of judgments and advisory opinions of the Inter-American Court of Human Rights, in light of the Vienna Convention of 1969 and related theories on treaty interpretation.