The purpose of this study was to examine the second language development of a native-speaker of Spanish learning English over a period of 15 months. More specifically, I explored the interaction of complexity (advanced forms of grammar and vocabulary), accuracy (grammatical and semantic), and fluency, commonly referred to as the CAF constructs. While findings in CAF literature tend to focus on one construct using experimental or cross-sectional studies (Bulté & Housen, 2012; Kormos & Dénes, 2004; Vyatkina, 2012), this case study investigated non-linear and interconnected CAF development, periods of fluctuation, and the effects of motivational factors on 14 variables. In order to explore the data as a system developing simultaneously, Complex Dynamic Systems Theory (CDST) (Larsen-Freeman, 1997; 2006) was applied as the theoretical framework. Through CDST’s theoretical lens and the tools developed for it (Verspoor, de Bot & Lowie, 2011), I found that knowledge variables (lexical diversity, accuracy, and elaboration) maintained consistent correlations, whereas their relationship with fluency variables (speed, repairs, and pauses) changed based on the cognitive strain the learner was experiencing at the time. I also found that the learner shifted his focus between the knowledge variables and that the complexity and accuracy variable on which he chose to focus appeared to be affected by changing motivational factors. / Applied Linguistics