This chapter focuses on the mechanism that determines the distribution of clitic insertion (CL-insertion) in a set of well-known argument alternations in Spanish (reflexives, anticausatives, and passives/impersonals). We claim that a clitic is inserted post-syntactically when v([EXT ARG]) is not merged with a DP in the syntax. The absence of a Spec, vP is due to the lack of phi-features on C or v. By adapting Muller's (2010) proposal that Merge is induced by subcategorization features, we argue that these features must be discharged at the morphological level for well-formedness conditions. If some of these features are not satisfied in the syntax, CL-insertion applies as a repair strategy to remove the surviving subcategorization feature. This is precisely the case in the empirical domain with which we are concerned: a v([EXT ARG]) that is not merged with any DP in the syntax induces CL-insertion at PF as a last resort operation. Such an operation is subject to strict locality conditions that determine morphological visibility for cyclic computation (Embick, 2010; Marvin, 2002). We will show that a restrictive theory of CL-insertion not only derives the basic distribution of some argument alternations, but also accounts for why CL-insertion can rescue external arguments but not internal ones. If our approach is on the right track, it follows that the notion of argument structure should be considered as an epiphenomenon and not as primitive of grammatical theory.