This book examines intra-individual variability and inter-individual variation in the emergence of syntactic complexity, accuracy, lexical complexity, and fluency (CALF) in L2 English writing at secondary school from the perspective of complex dynamic systems theory (CDST). It presents a mixed-method study based on the Written English Developmental Corpus of Polish Learners (WEDCPL), which consists of 1924 texts written by 100 learners during 21 repeated measurements conducted over a period of three years at secondary school. The study provides both group and individual data on language development, dynamic relationships between selected variables, significant peaks and phase transitions, patterns of intra-individual variability, the growth rate of language development, and the relationship between intra-individual variability and the average semester growth rate of language development. The book is addressed to SLA researchers, methodologists, foreign language teachers, and students.