This chapter introduces the basic concepts of complex systems as an important step in mapping the interdisciplinary space, finding out the link between complexity and communication networks. As the book aims to explore how emerging technologies, such as artificial intelligence, machine learning, and enabling paradigms for 6G, intersect with complex systems, a solid understanding of their main features is essential. Complex systems theory offers powerful tools to model emergent behaviors, nonlinearity, and adaptive dynamics, which increasingly shape the design and behavior of modern communication infrastructures. This chapter enables readers to recognize how complexity science can reshape key aspects of networks. The discussion covers perspectives, key mathematical formalisms, and the evolution of complexity as a transdisciplinary paradigm. This chapter sets the stage for understanding how complexity can lead to novel approaches in designing and managing next-generation communication systems.