Dynamic Systems Theory investigates complex systems and describes how these systems change over time. The transformation of complex, dynamic systems is never linear in fashion. They behave chaotically and their development is thus unpredictable. A minor change in the initial phases can have major consequences in the future. Therefore, such chaotic behaviour makes it impossible to predict how a transition from one state to another will proceed. Furthermore, when considering complex systems we cannot treat them separately from one another because all their parts are interconnected. A change in behaviour in one system by necessity leads to the reorganization of others. There is also considerable variation both within several systems and between them. In developmental studies intra-individual variability indicates that a system is undergoing dynamic change and is far from being in a state of equilibrium. Variation between systems, on the other hand, suggests that they are in different phases of development.

Because Dynamic Systems Theory focus on the behaviour of complex systems and describes how they change over time it has also proven useful in second language studies. In this area it investigates how a new language emerges in second language learners. Hitherto studies, conducted from the point of view of DST, have investigated second language development in a few learners and with a dense data collection point. What was missing, however, were longitudinal studies analyzing a larger set of individuals, where the focus is not on investigating different groups at different levels, but rather on tracing the development of the individuals forming a single group. The present study was conceived with a view to filling this gap. Its aim was to follow the development of fifteen young adults with Polish as their L1 who were beginning to learn a new language (Swedish) in a classroom environment. Their development was followed in all stages, starting from the first semester and ending with the end of the third year of learning Swedish. The study was not microdevelopmental but rather macrodevelopmental in character due to the fact that experimental sessions took place at the end of every semester, i.e. after about every 120−150 hours of second language classes.