Constitutional interpretation is a very complex task. The main reason underlying this complexity is the open and abstract language of constitutional texts, mainly when it concerns their bill of rights. And when it comes to judicial review of legislation, constitutional interpretation becomes even more complex. Not only the constitution but also ordinary legislation has to be interpreted so that their compatibility can be properly analyzed. Although this scheme represents common sense among constitutional scholars, the arguments used in the judicial review are the subject of fierce disputes. The aim of my paper is to analyze one of these arguments, which is frequently employed in Latin American constitutional adjudication: the presumption of constitutionality. I will argue that this kind of presumption entails many problematic issues of which constitutional scholars in Latin America are often unaware. Roughly speaking, these problematic issues can be of two types:(1) Formal argumentation problems - concerning above all the relationship between presumption and time, as well as between presumption and proof; and (2) Constitutional theory problems - concerning some consequences of the presumption of constitutionality in the separation of powers.