Since seminal work in the late ‘80s the general consensus in the literature is that children who are regularly exposed to two languages from birth or soon after can acquire them as largely independent systems. In recent years the focus has shifted from the issue of language differentiation to the degree of interaction between the two systems, and to the factors affecting such cross-linguistic influence. The aim of this paper is to provide an overview of current proposals about the determinants and the manifestation of language interaction in young simultaneous bilinguals.