This study concerns the acquisition of complex sentences with perception and epistemic verbs that take a second verb in their complements. The acquisition of complementation began between two and three years of age in this longitudinal study of four children's spontaneous speech. The results of the study showed that (1) complement types and complementizer connectives and (2) the discourse contexts in which complementation occurred were specific to individual matrix verbs. The most frequent verbs acquired were the perception verbs see and look and the epistemic verbs think and know. Developments in both discourse and syntax indicated that these verbs expressed attitudes of certainty/uncertainty toward the content expressed in their complements. The results are discussed in terms of both linguistic and psychological factors in the acquisition of complex sentences with complementation.