This dissertation investigates the difference between phi-agreement and clitic doubling using Tunisian (Maghrebi) and Palestinian (Levantine) Arabic as an empirical ground: I look at the same series of morphophonological clitics in both dialects, in four different contexts, and show that we can distinguish two syntactic types: Doubling clitics and agreement clitics. I defend a tripartite taxonomy of clitics, where doubling clitics are neither pronouns nor agreement, but at a discrete stage on the grammaticalization path between the former and the latter. I analyze doubling clitics as heads that are part of the extended projection of the verb, with the clitic head CL as a μ-binder, requiring an element in its specifier binding a trace or a pronoun. By contrast, pronominal clitics are Ds with an elided NP complement, and agreement clitics are the realization of a phi-probe in T.