The present paper reviews the literature on issues related to multilingualism and foreign language development. It starts with psychological factors involved in second language acquisition; then, it examines studies which looked into developmental aspects of language. It also reviews studies that compared second and foreign language development. Different cognitive factors implicated in learning additional languages are tackled. For instance, Lenneberg’s hypothesis of a critical period in addition to other factor like language attitudes are all dealt with in this paper. There are two main objectives behind this review: first, it purports to examine the variables which have been focused on in the literature and the gaps, if any; second, it aims to provide a framework for the understanding of foreign language development. It also provides useful insights into the topic of language development in general.