Differential marking as applied to direct objects has long been discussed as one of the characterizing traits of many Romance languages. There is, however, wide consensus that a detailed investigation into the nature of this phenomenon raises numerous challenges both at the empirical and theoretical level. Many questions are still being raised regarding which precise morpho-syntactic strategies count as differential object marking, whether the data can be unified, and, subsequently, how they are to be unified formally and theoretically. Additionally, a thorough investigation of this phenomenon is still needed for many Romance languages and especially at the micro-variation level. This volume brings together original papers addressing various aspects of differential object marking in Romance languages, focusing on micro-variation, from both a descriptive and formal perspective, touching on diachrony, language contact, synchrony, and using a large set of methodologies.