Complex systems theory (also known as theory of complexity or complexity perspective) is partly derived from general systems theory (von Bertalanffy 1968; Warren et al. 1998). It focuses on understanding complex systems or complex adaptive systems. Complex systems often encompass a (often large) number of entities and subsystems, among which one often observes multiple interactions, nonlinear relationships, feedbacks, thresholds, lags and adaptations. As a result, these features in complex systems can lead to emergent phenomena or results that are not analyzable solely from system components and their attributes.

The modeling of complex systems is defined by the application of various techniques math, statistics and computing, to generate insight into how some of the world's most complex physical and natural systems work.

Complex systems are those whose properties result from the nonlinear interaction of components, such as the cells of a body, the neurons of a brain or the individuals of a group. Our technical approaches run the gamut from very simple deterministic and stochastic mathematical approximations, through principle-based approaches to dealing with uncertainty, to complex computational and physical simulation in software and robotic hardware. Given their ubiquity, it is difficult to overestimate the benefits of a better understanding of complex systems and the appropriate tools to analyze them.

Understanding and managing complex systems has become one of the greatest challenges for research, policy and industry. The modeling and simulation of complex systems promise to allow us to understand how a human nervous system and brain not only maintain the activities of a metabolism, but also enable the production of intelligent behaviors, how huge ecosystems adapt to changes, or what actually influences climate change. 

In addition, artificial systems are becoming more and more complex and difficult, if not impossible to understand. Therefore, we need methods and tools that can help us, for example, to estimate how different infrastructure investments will affect the transport system and to understand the behavior of large internet-based systems in different situations. This type of system becomes the center of research and sustainable management because there are now techniques, tools and IT resources available.