Chapter 9 presents the results of the case study, the aim of which was to examine intra-individual variability and inter-individual variation in the emergence of syntactic complexity, lexical complexity, accuracy, and fluency (CALF) in L2 English writing at secondary school in the case of selected good, average, and poor language learners. The results of the case study showed that the individual learners did not differ much from one another and the group with respect to average results on syntactic complexity, lexical complexity, accuracy, and fluency. However, the learners differed in terms of significant progress, prioritising different language subsystems. Moreover, they created diverse relationships between the CALF variables which changed over time. Their development of the CALF measures was not more variable than in the group, but their relationships between the variability patterns were different. The individual learners demonstrated few significant peaks and phase transitions in CALF measures, but they developed these measures at different rates per semester and over the whole educational level. Generally speaking, the good, average, and poor learners did not follow learning patterns that could be said to be characteristic of a given category. However, intra-individual variability had a positive influence on the average semester growth rates (ASGRs) of most of the variables.