Chapter 4 describes complexity, accuracy, and fluency (CAF) in the development of L2 writing. It addresses the three issues combined in the present research project, namely, the writing process, the CAF triad, and learner corpora. The writing process is described by means of the cognitive model of L1 writing (Hayes & Flower, Identifying the organization of writing processes. In L. Gregg & E. Steinberg (Eds.), Cognitive processes in writing: An interdisciplinary approach (pp. 3–30). Lawrence Erlbaum, 1980), its revised versions, such as the individual-environmental model (Hayes, A new framework for understanding cognition and affect in writing. In C. Levy & S. Ransdell (Eds.), The science of writing: Theories, methods, individual differences, and applications (pp. 1–27). Lawrence Erlbaum, 1996) and the collaborative writing model (Leijten et al., Journal of Writing Research 5(3):285–337, 2014), as well as the application of the original cognitive model to L2 writing (Wang & Wen, Journal of Second Language Writing 11(3):225–246, 2002; Zimmermann, Learning and Instruction 10(1):73–99, 2000). The differences between novice and expert writers are explained in terms of the knowledge-telling and knowledge-transforming models (Bereiter & Scardamalia, The psychology of written composition. Lawrence Erlbaum, 1987). The elements of the CAF triad are described in terms of the underlying cognitive, linguistic, and psycholinguistic processes. A detailed taxonomy of complexity, accuracy, and fluency accounts for different measures and computer programmes used in their analysis. Finally, the chapter defines learner corpora with reference to design criteria, presents a typology of learner corpora, and describes methods used to analyse them. It provides an overview of learner corpus research (LCR) in SLA and ELT, including its critical evaluation, and offers suggestions for further development.