There is a growing recognition that many problems facing policies and programmes are complex and need to be treated as such. Implementation must deal with interdependent problems, navigating nonlinear and often unpredictable change processes, involving a diverse range of stakeholders. The first half of this paper therefore aims to give readers the tools to decide in what way, and to what degree, the challenges they face are complex – and sets out the central reasons why complex problems present big challenges for traditional approaches to implementation: - Firstly, the capacities to tackle complex problems are often distributed among actors: problems manifest themselves in different ways and at different levels, and rather than one organisation or hierarchy being fully in control of meeting a particular objective, action may rely on differing degrees of collaboration from a variety of actors. - Secondly, complex problems are difficult to predict: many social, political and economic problems are not amenable to detailed forecasting. Where causality is not well understood success may rely on adaptation and flexibility to emerging insights, rather than trying to completely fix the shape of policy responses in advance. - Thirdly, complex problems often involve conflicting goals: there may be many divergent but equally plausible interpretations of a policy issue, with different groups approaching it from different starting points or assumptions. Implementation cannot be technocratic, but requires a negotiated understanding and synthesis through communicative processes.