This article illustrates that studying intra-individual variability in Second Language Development can provide insight into the developmental dynamics of second language (L2) learners. Adopting a Dynamic Systems Theory framework (Thelen & Smith, 1994; van Geert, 1994) and using insights from microgenetic variability studies in developmental psychology (Siegler, 2006), we focus on L2 systems during a time of rapid development, applying advanced visualization techniques. A reinterpretation of a longitudinal study by Cancino et al. (1978) on the use of negation shows nonlinear patterns and peaks of regression, and illustrates the relevance of regarding internal variability as a source of information in itself. A case study of an advanced learner reveals a general increase over time for the correlates included, but the development is nonlinear, showing moments of progress and regress. The case study also brings to light an interesting dynamic interaction of subsystems. In accordance with the assumption of a limitation of resources, the learner shows a variable development for some related measurements in the course of the trajectory.