This book investigates bilingual speech production and the underlying cognitive processes that facilitate target language selection, lexical access, and inhibitory control of cross-linguistic interference. In particular, it examines whether or not certain factors representing L2 proficiency such as linguistic background/bilingual type, age of acquisition, and verbal fluency modulate these processes. An exploratory study is presented in which language learners and heritage speakers participated in a variety of experimental tasks. A significant effect was found for L2 verbal fluency to the degree that less fluent bilinguals relied upon different cognitive mechanisms than their more fluent counterparts. The Selection by Proficiency Model is presented which illustrates the critical role of L2 verbal fluency on language selection, lexical access, and inhibitory control. This book discusses subsequent studies which have provided extensions to the model, particularly regarding the robustness of the L2 lexicon in order to formulate suggestions for future research.