Almost all interesting processes in nature and society are highly cross linked. In many systems, however, we can distinguish a set of fundamental building blocks, which interact nonlinearly to form compound structures or functions with an identity that requires more explanatory devices than those used to explain the building blocks.

Multivariate systems that need complementary, multi-level modes of description are defined as complex systems. They are typically modelled as networks or dynamical systems.

Examples abound:

• Gene networks that direct developmental processes under selective pressure;
• Immune networks that preserve the identity of organisms;
• Social insect colonies;
• Neural, physiological, and technological networks that produce intelligence;
• Ecological networks;
• Social networks comprised of transportation, utilities, and telecommunication systems, as well as economies and political deliberation processes.