Abstract: In this paper, I analyze null subjects in absolute clauses (AC) in Spanish, comparing them to finite-clause null subjects and infinitival subjects. AC null subjects mostly pattern like finite-clause null subjects, but they cannot be identified by a rich inflection and they require a grammatically encoded antecedent (like some infinitival subjects). I propose that AC null subjects must be identified by a topic operator through the operation of AGREE, a matching operation between two categories in which one of them copies its values to the other. AC null subjects remain unvalued in their own clause, seeking an antecedent in the preceding clause through a mediating null topic in the absolute clause’s periphery. Finite-clause null subjects, on the other hand, are identified within their own clause by verbal morphology through the same AGREE mechanism. Two distinct ways of relating null subjects to discourse-antecedents are proposed, one for finite-clause subjects, one for absolute-clause subjects. By proposing a distinct discourse-identification process not mediated by rich agreement, this proposal opens the door to a unified theory of null referential categories, in which discourse-topic identification plays a central role. [Copyright &y& Elsevier]