A central empirical fact about many expressions in natural language is that they involve, at some level of mental representation, arrangements of nested constituents that enter into relations of various kinds by virtue of being so arranged. A theory of phrase structure (PS) in the most general sense is a set of hypotheses about how this internal organization of expressions and the dependencies it supports are mentally represented as part of the speaker’s knowledge of language. This vignette focuses on the somewhat neglected question of the limits of PS theory. After demarcating, in most general terms, the analytical purview of such a theory, I will turn to a number of illustrative examples of constructions that have resisted a coherent description in PS terms, which I suggest may indicate that they fall altogether outside its scope.