The aim of this study is to provide a detailed account for the phenomenon of movement epenthesis in Italian Sign Language (LIS). LIS displays at least two cases of epenthesis of movement, one affecting signs that involve contact with the body, the other affecting signs that do not (i.e. signs articulated in neutral space). The two cases of epenthesis of movement receive a unified analysis, once the mechanism of selection of the plane of articulation is spelled out. The general phenomenon of movement epenthesis is captured by a formal approach within a constraint-based framework, such as the one developed first for American Sign Language (ASL) in Brentari (1998). Cases of movement epenthesis in ASL will be discussed and compared to cases of LIS epenthesis