Dynamic systems theory, a general theory of change and development, offers a new way to study first and second language development and requires a new set of tools for analysis of empirical data. After a brief introduction to the theory, this book, co-authored by several leading scholars in the field, concentrates on tools and techniques recently developed to analyze language data from a dynamic perspective. The chapters deal with the general thoughts and reasoning behind coding data, analyzing variability, discovering interacting variables and modeling. The accompanying How to sections give step-by-step instructions to using macros to speed up the coding, creating a dedicated lexical profile, making min-max graphs, testing for significance in single case studies by running simulations, and modeling. Example files and data sets are available on the accompanying website. Although the focus is on second language development, the tools are applicable to a wide range of phenomena in applied linguistics.