This article provides a comprehensive and critical synthesis of the methods utilized in studies investigating the role of language aptitude in second language acquisition (SLA). The synthesis is informed by sixty-five studies generated by a thorough search of the literature, three meta-analyses (Li, 2015, 2016, 2017), and a thematic issue of Studies in Second Language Acquisition (Li & DeKeyser, in press). The synthesis starts by identifying three categories of research investigating the role of aptitude in naturalistic learning, aptitude's associations with instructed learning, and the nature of aptitude pertaining to whether it increases with age and learning experience and how it is connected to other individual difference variables. The synthesis then presents an overview and critique of major measures of aptitude and discusses the construct validity of aptitude measures based on the principles of psychometric assessments. Specifically, the measures are scrutinized along the dimensions of reliability, content validity, divergent/convergent validity, and predictive validity. The content and measurement of implicit aptitude—a newly emerged construct in SLA—are highlighted. The synthesis proceeds to summarize and vet the measures of the outcome variable of aptitude research—L2 proficiency. Throughout the synthesis, methodological features are summarized, issues are identified, and remedies are proposed.