The research presented uses theoretical constructs of formal syntax as tools to capture performance data in agreement production. The phenomenon examined is interference in subject–verb agreement. A general overview of our experimental findings which highlights the different syntactic factors involved in interference is provided. A fine-grained characterization of the hierarchical structure is presented that involves 1) the structural definition of intervention, 2) the distinction between precedence and ccommand, 3) the role of intermediate traces, and 4) the role of subject movement. New data are presented that fit the theoretically motivated gradient of computational complexity proposed (Franck et al. 2006).