In this talk I will discuss the relationship between intonational phrasing and syntactic structure and argue that intonational phrasing preferences in language comprehension and production are related due to performance factors. In the area of production, researchers (Cooper & Paccia-Cooper, 1980; Gee & Grosjean, 1983; Ferreira, 1988) have attempted to describe the relationship between intonational phrasing and syntactic structure, and although these models enjoy varying degrees of success, all of them tend to be complicated and have large numbers of parameters. In light of these problems, I propose a simpler model called the Left/Right Constituent Boundary hypothesis (LRB). According to this hypothesis, two factors underlie the successful performance of previous models and contribute to the likelihood of producing intonational boundaries at word boundaries: 1) the size of the recently completed syntactic constituent at a word boundary; and 2) the size of the upcoming syntactic constituent. Two experiments will be presented that suggest that this hypothesis provides the best account of the data. In addition, I will argue that listeners exploit the relationship between intonational phrasing and constituency in production by using a special parsing heuristic. Listeners prefer not to attach incoming words to lexical heads followed by an intonational boundary. I will present data from a series of comprehension experiments that suggest that this hypothesis provides the best account of listener intonational boundary preferences.