 Alchourrón’s main line of research concerned the theory of belief change, which basically deals with the question of how a belief set is to be updated in light of new information. In 1985 he published, together with Peter Gändenfors and David Makinson, a seminal article in which they advanced an axiomatic theory that has come to be known as “the AGM theory of belief change”, “the AGM account of the logic of belief change”, or “the AGM theory of belief revision” (Alchourrón, Gändenfors, &amp; Makinson 1985; cf. Alchourrón &amp; Makinson 1982, 1985, 1986). The AGM theory identifies three types of belief change: expansion, contraction, and revision, focusing on the last two. Roughly put, expansion consists in adding to a given belief set a new belief that does not conflict with the current beliefs of the set; contraction involves removing a belief from the set; and revision amounts to adding a new belief to the set and removing others from it so that the resulting set stays consistent. The AGM theory proposes six basic and two supplementary postulates for each of the two operations of contraction and revision that every appropriate belief-change method must satisfy. Since its appearance in 1985, the AGM theory has had a tremendous impact on subsequent discussions of belief change, becoming the predominant paradigm, even though some of its main assumptions have been questioned and several modifications or extensions of it have been proposed (for an overview, see Arló-Costa &amp; Fermé 2010 and Hansson 2011; see also Fermé &amp; Hansson 2011, which is a special issue on the occasion of the 25 years of the AGM theory). The theory has also had a considerable influence on research on artificial intelligence (on which see Carnota &amp; Rodríguez 2010). Towards the end of his life, Alchourrón’s work focused on the logic of defeasible conditionals (Alchourrón 1993, 1995, 1996; see Fermé &amp; Rodríguez 2006 for an analysis of his theory of conditionals).