Systems theory or general systems theory or systemics is an interdisciplinary field which studies systems as a whole. Systems theory was founded by Ludwig von Bertalanffy, William Ross Ashby and others between the 1940s and the 1970s on principles from physics, biology and engineering and later grew into philosophy, sociology and economics. Cybernetics is a related field, sometimes considered as a part of systems theory.
Systems theory focuses on complexity and interdependance which increasingly are the features of the modern world, and thus systems theory has a strong philosophical dimension. In recent times complex systems has increasingly been used as a synonym.
Part of systems theory, system dynamics is a method for understanding the dynamic behavior of complex systems. The basis of the method is the recognition that the structure of any system -- the many circular, interlocking, sometimes time-delayed relationships among its components -- is often just as important in determining its behavior as the individual components themselves. Examples are chaos theory and social dynamics.
In recent years, the field of systems thinking has been developed to provide techniques for studying systems in holistic ways to supplement more traditional reductionistic methods. In this more recent tradition, systems theory is considered by some as a humanistic counterpart to the natural sciences.