In this contribution, some of the basic characteristics of complex adaptive systems, collectively labeled Dynamic Systems Theory (DST), are discussed. Such systems are self-organizing, dependent on initial conditions, sometimes chaotic, and they show emergent properties. The focus in DST is on development over time. Language is seen as a dynamic system, and language development, both acquisition and attrition, as a dynamic process. A number of examples of possible applications of DST in the field of applied linguistics are mentioned. After a short presentation of each of the individual articles, some possible lines of research are discussed.