Chapter 5 deals with (asymmetric) differential object marking (DOM), a phenomenon occurring in over 300 languages whereby the object of a verb can be either marked or unmarked for morphological case, depending on a number of factors. These factors have to do with the individuation/prominence of the object, often its definiteness, specificity and animacy value. The chapter begins with illustrations of the phenomenon in several languages, showing its sensitivity to different properties of the object. The reasoning behind DOM is then discussed. Two specific proposals are considered: one treats DOM as a signal of higher transitivity; the other views it as a disambiguation mechanism whereby a non-prototypical object is marked and, as a result, not confused with the subject. In addition to features of the object, properties of the verb have been argued to play a role in DOM; this relation is considered in a separate section. Further, several cross-linguistic analyses of the phenomenon are reviewed.