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 (http://dx.doi.org/10.1075/lllt.29.website). Although the focus is on second language development, the tools are applicable to a wide range of phenomena in applied linguistics.