This edited volume is a timely and significant contribution to second language development research from a Complex Dynamic Systems Theory (CDST) perspective. The book addresses the growing need for consistent and practical methodologies for CDST research. Each chapter presents an innovative method or approach, illustrating its use in concrete applications to empirical studies and highlighting its value in capturing the dynamic, time-sensitive, and interconnected nature of L2 development. The volume emphasizes a shift from static outcomes to developmental processes, offering tools to explore intra-learner variability, network analysis, dynamic modeling, and more. By bridging theory and practice, it equips researchers with methodological guidance to investigate the process of second language development. Building on and extending previous volumes on CDST research methods by Verspoor et al. (2011) and Hiver and Al-Hoorie (2019), this book serves as a crucial reference for future CDST-informed research in Applied Linguistics.