The five contributions describe some key mathematical concepts involved in the study of complex systems and non-perturbative problems. The selection of topics is intended to cross-fertilize the various fields where complex systems theory has made an impact. The book presents specific and detailed results meant for a wide audience of researchers and students. It begins with those contributions which help to set up a general theoretical framework and ends with selected applications to the particular areas of biophysics, statistical physics, astrophysics and cosmology. It also includes an extensive bibliography. This pedagogically written text can be used as an introduction to the fundamental ideas behind complex systems theory.