This article discusses research on young children's comprehension of relational terms such as BEFORE, AFTER, BECAUSE, and SO. A general review of the literature on children's comprehension of these terms is followed by a critique that is supported from three sources - the author's own data on children's accurate production of the same terms they apparently fail to comprehend, methodological critiques of the interpretation of the research on children's comprehension, and a theoretical account of the comprehension-production disparity in terms of the differing cognitive demands of comprehension and production, and of different types of comprehension stimuli. Comprehension of relational terms involves more than comprehension of a single term; comprehension involves forming a mental representation of the relationship between two propositions that is expressed by the term. Forming a mental representation of a logically constrained or familiar relationship would seem to be cognitively simpler than forming a mental representation of an arbitrary or unfamiliar relationship. Two studies were carried out to assess this proposal, one assessing comprehension of sentences containing BEFORE and AFfER, the other assessing comprehension of sentences containing BECAUSE and SO. For each study, older children were equally able to comprehend the tenns regardless of the semantic content of the propositions being related, but the performance of younger children was strongly influenced by semantic content. The younger children gave evidence of having full knowledge of the lexical components of the terms being tested, but were not able to apply this knowledge in order to form mental representations of (comprehend) novel relationships. These results are discussed in terms of young children's knowledge being closely tied to personal experience, and development consisting in part of the ability to move beyond the constraints of operating within familiar cognitive contexts.