Despite the complexity and interconnectedness of problems faced in humanitarian and development work, they are often approached in an overly simplistic manner, informed by linear ways of thinking. This paper draws on the science of complexity to outline an alternative approach to analysing and understanding these problems. It explores and explains ten key concepts of complexity science and considers their implications for development and humanitarian work. It concludes that complexity science can enable those thinking about and working on these problems to better understand and adapt to the complexities of the real world. Its findings will be of particular interest to those working on processes of change - be they researchers, policy analysts and advisors, evaluators or managers who are working on change and reform initiatives.