This paper presents a process model of sentence comprehension. Drawing upon a modular conception of the language apparatus, the model distinguishes several levels of structural representation as well as several special-purpose processing mechanisms, including the verbal working memory system. The perspective gained from considering the functional architecture of language processing puts us in a position to advance specific hypotheses about the causes of impaired performance in populations with notable language difficulties. To test among these hypotheses, a methodological prescription is given for disentangling the subcomponents of the language apparatus which are intertwined in ordinary language use. This prescription is followed in an attempt to uncover the cognitive and linguistic processes implicated in reading disorder. We then proceed to consider empirical findings on both normal children and children who are reading disabled. The results lend support to the view that comprehension failures by poor readers arise from limitations in language processing involving working memory. This conclusion challenges the hypothesis that their reading difficulties reflect a developmental lag in the acquisition of syntax.