We discuss theoretical approaches to blocking effects, with particular emphasis on cases in which words appear to block phrases (and perhaps vice versa). These approaches share at least one intuition: that syntactic and semantic features create possible “cells” or slots in which particular items can appear, and that blocking occurs when one such cell is occupied by one form as opposed to another. Accounts of blocking differ along two primary dimensions: the size of the objects that compete with one another (morphemes, words, phrases, sentences); and whether or not ungrammatical forms are taken into consideration in determining the correct output (relatedly, whether otherwise well-formed objects are marked ungrammatical by competition). We argue that blocking in the sense of competition for the expression of syntactic or semantic features is limited to insertion of the phonological exponents of such features (the Vocabulary items of Distributed Morphology) at terminal nodes from the syntax. There is thus no blocking at the word level or above, and no competition between grammatical and ungrammatical structures. The architectural significance of these points is emphasized throughout the discussion.