Complex dynamic systems theory (CDST) provides a cognitive approach to variability which contrasts with earlier linguistic, sociolinguistic, and psycholinguistic approaches in second language acquisition (SLA). According to CDST, intra-individual variability, defined as the differences in the level of a developmental variable within individuals upon repeated measurements, is said to drive language development. So far, the role of intra-individual variability in language development has been examined mainly in longitudinal case studies, involving few participants, at the university level. The aim of this monograph is to present a longitudinal mixed-method study, consisting of a panel study and a multiple-case study, the aim of which was to examine the role of intra-individual variability in the emergence of syntactic complexity, accuracy, lexical complexity,, and fluency (CALF) in L2 English writing at secondary school. The study was conducted in 2014–2017 at a secondary school in Czestochowa, Poland. It was based on the Written English Developmental Corpus of Polish Learners (WEDCPL), which consists of 1923 essays written by 100 learners during 21 repeated measurements, conducted over a period of 3 years (CECL, Learner corpora around the world. Université catholique de Louvain, 14 October 2024). The results of the study partly align with previous research findings and provide practical implications for language teachers.