Complex systems are difficult to describe and predict because they have many interconnected parts with emergent properties.

They may, under certain conditions, behave in a chaotic way. Chaos is normally understood to be a state of utter confusion or disorder but formal chaos theory is the study of non-linear systems, which show great sensitivity to initial conditions.

Complexity theory is the study of how both complex and complicated systems exhibit simple behaviours. High complexity in a project or organisation is associated with tight spacing of equipment, close production steps, many common mode connections, a limited possibility of isolating failed components, limited awareness of interdependencies because of specialisations, limited ability to substitute, unintended feedback loops, interacting control parameters, limited understanding of social processes and tightly coupled systems. A complex system is not just complicated but may be incomplete, with emergent properties from interdependencies that are unknown and unforeseen. They often cannot be ‘solved’; rather, they have to be managed to desirable outcomes.

A complicated system may contain lots of elaborately interconnected sub-systems or holons. To the non-expert, complicated systems may seem complex, but to an expert each of the subsystems may be tame (i.e.  understandable and predictable). There may be sufficient experience and knowledge for dependable design (e.g. for the jet turbine).

Evolution in complex systems leads to differentiation in multi-level hierarchies. A hierarchical model is a graphical representation of a concept or idea in the form of a hierarchy. We develop multi-level hierarchies of processes using mind maps.