This paper addresses two issues, the categorial nature of (pronominal) clitics and the conditions ruling their surface distribution as proclitics and enclitics, which we take as related. We claim that pronominal clitics are defective functional heads consisting of a bundle of φ-features, and hence, that they are merged in a fixed position in the functional structure of the clause. The (micro)variation in clitic order patterns will be accounted for through the interplay of verb movement and the Proclisis Parameter: when languages set the value ‘yes’ for this parameter, proclisis is the dominant pattern, whereas in languages setting the value ‘no’ for the same parameter, enclisis dominantly occurs; other differences between Romance languages and, in particular, EP and BP, are accounted for by the properties of the nodes T and Asp, namely, their ability to attract V and or to check uninterpretable features through Agree without Attract.