An emerging field of Systems Science studying systems that exhibit emergent behaviour. The behaviour of complex systems is not easily inferable from its properties.
Complex systems have the following properties:
Any modeling approach that ignores the difficulties of predicting complex system behaviour or characterizes them as noise will necessarily produce models that are neither accurate nor useful. As yet no fully general theory of complex systems has emerged for addressing these problems, so researchers must solve them in domain-specific contexts.
Examples of complex systems are Earth's global climate, organisms, the human brain, infrastructure such as power grid, transportation or communication systems, complex software and electronic systems, social and economic organizations (like cities), an ecosystem, a living cell, and ultimately the entire universe.