Medieval French, usually analyzed as a null subject language, differs considerably from modern Romance null subject languages such as Spanish in the availability of non-expressed subject pronouns; specifically, it shows characteristics reminiscent of non-null, rather than null subject languages, such as the expression of expletive subject pronouns. The central goal of this book is to put forward an account of these differences. On the basis of the analysis of an extensive, newly established data corpus, the development of the expression of both expletive and referential subject pronouns until the 17th c. is determined. Following a thorough discussion of previous approaches, an alternative approach is presented which builds on the analysis of Medieval French as a non-null subject language. The non-expression of subject pronouns, licit in specific contexts in non-null subject languages, is shown to be restricted to configurations generally involving left-peripheral focalization. These configurations - and, concomitantly, non-expressed subject pronouns - are finally argued to be eventually lost for good in the wake of the initial observation by 17th c. writers of pertinent instructions campaigned for in highly influential works of language use.