Research on language comprehension has focused on the resolution of syntactic ambiguities, and most studies have employed garden-path sentences to determine the system's preferences and to assess its use of nonsyntactic sources information. A topic that has been neglected is how syntactically challenging but essentially unambiguous sentences are processed, including passives and object-clefts--sentences that require thematic roles to be assigned in an atypical order. The three experiments described here tested the idea that sentences are processed both algorithmically and heuristically. Sentences were presented aurally and the participants' task was to identify the thematic roles in the sentence (e.g., Who was the do-er?). The first experiment demonstrates that passives are frequently and systematically misinterpreted, especially when they express implausible ideas. The second shows that the surface frequency of a syntactic form does not determine ease of processing, as active sentences and subject-clefts were comprehended equally easily despite the rareness of the latter type. The third experiment compares the processing of subject- and object-clefts, and the results show that they are similar to actives and passives, respectively, again despite the infrequent occurrence in English of any type of cleft. The results of the three experiments suggest that a comprehensive theory of language comprehension must assume that simple processing heuristics are used during processing in addition to (and perhaps sometimes instead of) syntactic algorithms. Moreover, the experiments support the idea that language processing is often based on shallow processing, yielding a merely "good enough" rather than a detailed linguistic representation of an utterance's meaning.