The phonological phrase is a unit in the perceived phonological structure of sentences, delimited by occurrences of perceived pause and delimiting the minimal intonation contour. In a generative grammar of English, pauses are predicted by occurrences of phonological phrase boundaries (PB's) which are defined in terms of the universal system of distinctive phonetic features. In order to discover the principles governing the assignment of PB's, it is necessary to distinguish between obligatory PB's, which generally occur at sentence termini and are determined entirely by the syntax, and the additional PB's which may appear through the application of rules that refer to phonological and stylistic as well as syntactic factors. This study is concerned with specifying the relationship between obligatory PB's and syntactic structure. A hypothesis concerning this relationship is stated in the form of the Obligatory Boundary Insertion (OBI) convention. The OBI convention states that PB's are inserted as leftmost and rightmost constituents of all sentences that appear as root sentences in any 'late' (postcyclic) syntactic structure, where 'root sentence' may be defined as any sentence which is not included in a 'predicative sentence,' i.e., a sentence which contains VP as an immediate constituent. The OBI convention predicts that any element which is extraposed from a root sentence will be separated from the root sentence by pause. Likewise, any sentence which is transformationally inserted into a root sentence from outside will be set off by pause. It is predicted that no pause will be introduced as the result of movement of constituents within the confines of a single predicative root sentence. A number of constructions resulting from the application of movement transformations are examined, and it is shown that in the cases examined the phrasing either supports or is at least consistent with the hypothesis. The investigation leads to a reanalysis of several transformational processes, including topicalization, subject-auxiliary inversion, right-dislocation, parenthesization, and secondary conjunction. Some evidence arises which suggests that the OBI convention should be generalized so as to assign PB's not only to root sentences but also to all phrases which in derived structure are not contained within predicative sentences. The principles of phonological phrase boundary insertion investigated here are of significance for the further study of the rules of English stress and intonation. An explicit and testable hypothesis concerning the relation between syntactic structure and phonological phrasing can also serve as an important criterion for the analysis of derived constituent structure and the rules which generate it.