A complex system is composed of interconnected parts (components) that are either complex systems themselves or capable of exhibiting global behavior not displayed by the lone components themselves. A typical complex system may be self-organized, nonlinear, stochastic, far from equilibrium, networked, globally coupled, with long-range interactions, with competitive and noncompetitive decision makers, etc. Thus the complexity of such systems is unavoidable in real-world problems. This chapter provides a brief background on complex systems and tools from control systems theory that will be used in subsequent chapters to solve different interesting problems.