The science of complex systems is not an offspring of physics, biology, or the socialsciences, but a unique mix of all three. By recalling what these diciplines are, wedevelop an intuitive feel for complex systems and for how this science differs from otherdisciplines. This chapter aims to show that the science of complex systems combinesphysics, biology, and the social sciences in a unique blend that is a new discipline in itsown right. We review a few classical concepts of the science of complex systems andemphasize the importance of co-evolution of states of components and their interaction.The chapter clarifies the structure of the book.