The fluency task, which involves lexical retrieval from a categorical or thematic prompt, has been a crucial paradigm in studies about lexicon in bilingualism and additional languages. Particularly, there has been a very productive cognitive turn towards analytical models based on the exploration of the cognitive processes underlying the mental lexicon using this methodological procedure. The aim of this chapter is, firstly, to review the theoretical foundations and concepts that inform this line of research, particularly those focused on lexical availability in English and Spanish as additional languages. Secondly, the most relevant studies of the past decade adopting an interdisciplinary cognitive approach are addressed, highlighting research that identifies cognitive factors related to availability, explores types of semantic relationships between words, and focuses on the creation of semantic networks. [For the complete volume, "Lexicon Exploration in English and Spanish Learners. Educational Linguistics. Volume 67," see ED676344.]