This study compared sentence comprehension skills in typically developing children 5–17 years of age, children with language impairment (LI) and children with focal brain injuries (FL) acquired in the pre/perinatal period. Participants were asked to process sentences ‘on-line’, choosing the agent in sentences that varied in syntactic complexity (actives, passives, subject clefts and object clefts), and in the presence or absence of a subject-verb agreement contrast. Results revealed that accuracy and processing speeds vary with syntactic complexity in all groups, reflecting the frequency and regularity of sentence types. Developmental changes continued throughout childhood, as children became faster and more accurate at processing more complex sentence structures. Children with LI and children with FL were quite profoundly delayed, displaying profiles similar to, or more impaired than those of younger children, but there was no evidence in the FL group for a disadvantage in left- vs. right-hemisphere-damaged children. Children with LI showed one unique pattern: higher than normal costs (reflected in reaction times) in using converging information from subject-verb agreement, in line with studies suggesting special vulnerabilities in grammatical morphology in this group. Results are discussed in terms of the Competition Model, a theory of language processing designed to account for the statistical changes in performance that are observed during development, and the probabilistic deficits in children with language impairments.