Complexity theory has been embraced by a wide range of disciplines - physics, biology, earth sciences and economics, strategy development, urban planning and climate modelling to name but a view. It developed, through the 20th century, out of early systems thinking, non-equilibrium thermodynamics and evolutionary theory. But is there one theory or many? Are some of these applications more metaphorical than scientific? Can we deepen our knowledge by sharing practice and examples between disciplines?