Second language (L2) aptitude has been broadly defined as the rate and ease of initially acquiring a second language. Historically, L2 aptitude has been understood as a stable trait that predetermined L2 achievement, regardless of individual learners’ efforts to acquire an L2. This traditional view of L2 aptitude as fixed and stable has led to it being a relatively neglected area of research within second language acquisition (SLA) studies. The little research that was in fact conducted was diagnostic in nature, and mostly used tests such as the Modern Language Aptitude Test (MLAT) to select potentially gifted L2 learners. Given that six decades have passed since the publication of the MLAT, now is a good time to revisit the literature and investigate whether L2 aptitude continues to be viewed as an individual difference of little interest to SLA research. While summative literature reviews of L2 aptitude research have been written, few systematic reviews exist. This article conducts a systematic quantitative literature review (SQLR) to provide a principled, comprehensive and reproducible synthesis of research into L2 aptitude published over the last 60 years (1959–2019). In this SQLR, close to one hundred journal articles and PhD dissertations were examined to discern generalisations and limitations in the field. This SQLR identifies a shift in the rationale for L2 aptitude testing, in which a diagnostic focus has been replaced by an explanatory perspective. Furthermore, our article points to a renewed interest in L2 aptitude research, which has come to be characterised by a more nuanced and sophisticated understanding of the concept and its components.