Current theories of specific language impairment (SLI) in children fall into 2 general classes: those that attribute SLI to processing limitations and those that attribute the disorder to deficits in grammatical knowledge. In this study, the authors examined children's comprehension of subject and object Wh-questions because they offer the means of determining the relative contribution to performance of knowledge and of processing. Comprehension of subject and object Wh-questions presumably requires knowledge of grammatical movement. However, through manipulation of the length of the questions, it is also possible to vary the processing demands of the questions. If a simple deficit in knowledge of movement is involved, children should show poorer comprehension on object questions than on subject questions, regardless of sentence length. However, if processing limitations are involved, length should affect comprehension of object questions but not subject questions. Children with SLI and typically developing (TD) children matched on receptive vocabulary test scores participated in a comprehension task consisting of short and long subject and object Wh-questions. The two groups performed similarly on short questions, each showing high accuracy in both subject and object conditions. However, the children with SLI showed poorer performance on long object questions compared to long subject questions. They were also less accurate on long object questions than were children in the TD group. We argue that demands on linguistic processing abilities play an important role in the difficulties experienced by children with SLI.