This book is a study of the micro-variation in the realization of definiteness across languages belonging to the Balkan Romance family: Romanian, Aromanian, Istro-Romanian, and Megleno-Romanian. The definite article is a suffix in all of these languages, but nominal constituents show considerable variation with respect to the overt realization of the definite article: in some instances, the definite article is spelled out only once, in other situations it is spelled out multiple times, and in still other cases it can be phonologically null. Daniela Isac offers a unified analysis of these options based on a post-syntactic spell-out rule that specifies the conditions under which the definite article can be pronounced on various heads within the nominal constituent. Micro-variation in the patterns displayed by specific languages in this family is accounted for exclusively by lexicon-related differences (the feature specification of lexical and functional items may vary across languages) and by differences related to externalization (syntactic relations such as Agree may have various morpho-phonological overt expressions across languages). Crucially, the computational system is assumed to be invariant, a result that is consistent with the generative understanding of the knowledge and acquisition of language.