Variability is a typical and inherent property of complex and dynamic systems. As a follow-up to Chang and Zhang’s (2021) study on inter- and intra-individual variability in EFL learners’ listening development, the present longitudinal study focused on exploring the variability of participants’ language aptitude and working memory, two prominent cognitive variables of learner individual differences (LIDs). Three EFL learners participated in 10 observations spanning 43 months and were requested to complete the IELTS, LLAMA, and a listening span test (LST). Time-series data of learners’ language aptitude and working memory were analyzed through CDST techniques such as moving min–max graphs and Monte Carlo Analysis. In our study, considerable and significant variability in language aptitude and working memory were identified not only between different learners but also within each learner’s dynamic learning process at different levels. The chapter concludes with a discussion on the importance of researching L2 learners’ inter- and intra-variability in understanding the dynamic nature of language aptitude and working memory from the CDST perspective.