Nowadays research in all disciplines is becoming increasingly interdisciplinary in character. Scholars utilise knowledge from other disciplines not only in order to better understand the phenomena they are investigating but also to obtain a different perspective, which always makes it possible to solve problems and find answers to questions in every sphere of life. This tendency is remarkable not only in the formal and natural sciences but also in the humanities. In the last few years a new approach, called Dynamic Systems Theory, has rapidly increased in popularity in Second Language Studies. This theory has in fact a long history, going back to Newton's Laws of Motion, and hitherto it has been used primarily in, among others, mathematics, biology and economics. As it investigates complex and changing systems DST can easily be adopted in Linguistics, especially in Second Language Development, where the focus is on complex and variable systems such as the second language learner, the (second) language, the learning environment etc.

When these complex systems develop, they are constantly changing and reorganizing so that in fact we cannot predict when a learner will achieve a certain level in the second language. Dynamic system behaviour is also characterised by considerable variability and they continuously interact with one another. Even a small trigger in the initial phases of the learning period can have a substantial impact on future development, which is comparable to the widely known butterfly effect. Additionally, significant differences always exist between second language learners who themselves are constantly changing. There is also variability within every student. All these properties are basic pre-requisites for investigating second language development from the point of view of dynamic systems theory.

In the present book we will look at the development of Swedish as a second language in young adults, beginning from the first months of their second language instruction and ending after three years of intensive language courses. During the course of this study we will focus on three proficiency dimensions: Complexity, Accuracy and Fluency and their interplay not only at group level, but also, and most importantly, in individual students. Chapter 2 provides an introduction to dynamic systems theory and its implementation for developmental and second language studies. Chapter 3 offers an overview of research conducted on the concepts of Complexity, Accuracy and Fluency in Second Language studies.