To date, the field of L3 acquisition research has had a heavy focus on the initial state of the L3 grammar. While this initial state research is critical to understanding L3 acquisition as a whole, in order for an explanatory understanding of language acquisition in the multilingual mind, the field needs to expand its scope beyond this area of focus. This volume brings together L3 acquisition and multilingualism researchers in order to discuss the state of the field and introduce new ideas related to the development of post-initial L3 knowledge, and the relationship among languages in the multilingual mind. It includes contributions related to syntactic, phonological, and lexical development beyond the L3 initial state. The purpose of this volume is to expand the current academic discussions within this field, by emphasizing the role of the developmental process in an L3 context, with the hope of inspiring further research in this domain.