In this talk I propose a unification of several morpho-syntactic phenomena as instances of a default mechanism whose function is to spell out semantically empty sentence elements when these are required for satisfying purely formal grammatical requirements. The relevant requirements are of two general types: morphological (cf. traditional accounts of do-support) and syntactic (cf. English-type resumptive pronouns that rescue island violations). I first review some default-based accounts of functional elements that I have argued for in earlier work: default agreement inflection (e.g., in Icelandic, Belfast English, Standard English) and default morphological case. In the main part of the talk, I argue for extending this notion of default to lexical heads (N, V, A, P) that lack intrinsic semantic content (e.g., "one", "be"). I claim that such elements have a last resort character, i.e., they can occur only when necessary. I show how this approach captures a wide range of facts, and explore its consequences for the architecture of the grammar, following the general framework of Distributed Morphology (Halle & Marantz 1993).