This chapter discusses the link between working memory, phonological short-term memory and language aptitude and describes how these cognitive abilities influence second language-learning processes. I provide a critical review of the definitions and constructs of aptitude and elaborate on how phonological short-term and working memory and components of language aptitude might influence processes of language learning, such as noticing, encoding in long-term memory, proceduralization and automatization, and aid second language processing and production. The chapter also considers the stability of cognitive variables in the course of language learning and presents evidence that certain components of language aptitude are prone to change with intensive exposure to second/third languages.