A complete theory of L2 (second language)acquisition should account for three aspects of L2 development: 1) the representation of L2 knowledge, 2) the relationship between input and L2 knowledge, and 3) the relationship between L2 knowledge and output. The research presented here is the first to analyze the L2 acquisition of object pronouns in Spanishwith such a tripartite framework in orderto identify an interface between the comprehension and production of the target structure throughout L2 development.