Chapter 3 focuses on the role of variability and variation in second language acquisition (SLA) and second language development (SLD). First, the chapter contrasts language systematicity, which refers to developmental stages in language acquisition, with language variability, which concerns the variable use of linguistic forms. Next, the chapter contrasts the cognitive approach to variability proposed by CDST with earlier linguistic, sociolinguistic, and psycholinguistic approaches in SLA. It presents a taxonomy of variability in learner language, followed by a detailed account of research on systematic variation, i.e., form-function, sociolinguistic, psycholinguistic, and functional variation, and on non-systematic or free variation, which is most important in CDST. Next, the chapter reports studies on intra-individual variability in motor, cognitive, and emotional development conducted in the field of developmental psychology. However, it concentrates on research findings concerning intra-individual variability and inter-individual variation in first and second language development. These research findings focus on variability patterns, significant peaks, phase transitions, and fractal coordination. Finally, the chapter proposes a number of descriptive and statistical methods recommended for examining intra-individual variability within the CDST paradigm. This includes the use of dynamic, non-linear, and stochastic models instead of static, non-linear, and deterministic models.