It is sometimes suggested that migration patterns evolve following a trajectory where individuals start to move from one country to another, and over time, more people join them: once a critical mass is reached, that migration flow expands rapidly. However, there is not enough evidence to back up this theory of migration dynamics partly because studies to date have tended to focus either on the ‘root causes’ of migration or on the reasons why migration processes gain momentum and become established migration systems. A related issue is lack of theory to explain why many initial pioneer migration movements do not set in motion self-reinforcing migration dynamics. Current theories also fail to explain adequately the stagnation and weakening of established migration systems. The THEMIS international project team investigated what makes people decide to migrate, why some of those initial moves to Europe result in the formation of significant migration systems, and why some migration processes simply tail off or stagnate. This involved a comparative study of the evolution of migrant groups following different migration trajectories from several regions of three origin countries (Brazil, Morocco and Ukraine) to selected areas in four destination countries (UK, Norway, the Netherlands and Portugal). Initiation, continuation and decline of European migration systems