This chapter begins with a discussion of systems theorySystems theory in relation to physical and biological systems and their philosophical foundations. From a philosophical point of view, systems theory is a form of structuralism that is oriented toward the concept of the “small world”. The extraction of “small worlds” in this context is part of a systematic and methodological approach: dissecting complex systems and isolating interacting elements, extracting their fundamental properties and causal interactions, and systematically reconstructing large systems from these simple interacting elements. In this chapter, we provide a conceptual overview of the benefits and challenges that the small-world approach brings to systems theory in general.