Clitic is an interface structure between semantic and syntax. In the same time, clitic has got both noun and verb features. Its syntax is linked with the theta-role configuration and its morphology is rich like a noun. The behavior of clitic has peculiar phenomena, i.e. climbing, and differences concerning the position of clitic and verb. There are many differences between proclitic (clitic before verb) and enclitic (clitic after verb). Classical generative syntactic models tried to explain these differences, but a best account of clitic phenomena is gift by OT models. In first and second chapters of my work, I introduce clitic in typology and the most relevant theories to justify its syntactic nature. Then, in third, 4th and 5th chapters, I present the work I did on CHILDES database data and my conclusion about children development of clitic use. Clitic in early italian L1 development shows a strong rule in proto-transitive structure. In particular, it is possible design three stages in development of clitic construction. In a first time (1;6-2;0), clitic is a frozen structure in a highly deictic item expression like “eccolo”; with a poor morphology and high frequency. Secondly (2;0-2;5), clitics show more morphology but the relative frequency decreases. At the same time, verbal island and full noun constructions increase. Finally, frozen structure “eccolo” is abandoned (2;5-4;0); the morphology is fully developed and pronominal and nominal structures have similar distribution. Moreover, at the and of 5th chapter, I analyze some substandard constructions in which clitics are used by children like a scaffolding structure for new lexical creations, e.g. “mi tanda delle stonni” (3;04).