System dynamics, a part of systems theory, is a method for understanding the dynamic behavior of complex systems. The basis of the method is the structure of a system with its many circular, interlocking, and often time-delayed cause and effect relationships between its parts. The nature of this structure forms a key characteristic of complex systems, emergence, which states that the characteristics of the whole are not from the separate parts, but from the interrelationships of those parts (see social system).