Complexity theory has its origins in the natural sciences where it has been applied to a wide variety of disciplinary fields, including neuroscience, ecology, epidemiology, computer science, and the study of physical phenomena such as turbulent fluids, gravitational systems in space, and to the intricacies of living cells. This chapter discusses nature of complexity theory and of complex systems. Complex systems are particularly sensitive to initial conditions that produce a long-term momentum or ‘path dependence’. Within economics, path dependency theory is used to illustrate how economies become locked into certain trajectories for sustained periods of time. Complex systems exhibit ‘emergence’ or behaviour that evolves from the interaction between elements. By way of contrast, ‘general complexity’ starts from an assumption of the multi-causal and non-linear nature of change and the importance of context. A further implication of a complex realist approach is a recognition of the role of values and emotions in the research process.