This article aims to analyze the legal distinction between expressing an opinion and making an assertion, as well as the legal consequences of both actions under Mexican law. Employing analytical, doctrinal, and jurisprudential methods, the study examines the concepts of opinion and assertion, along with their relationship to freedom of expression and defamatory statements. It argues that an opinion generally constitutes a constitutional protected subjective assessment, whereas an assertion -by presenting facts as true- can give rise to legal liability. Finally, the article analyses the limits of legislative immunity regarding opinions and assertions.