International migration is determined by both economic and political forces. This article examines the influence of economic, demographic and policy variables on British emigration to four principal destinations in two different eras. Before 1914 the economic and demographic forces that drove British emigration can be clearly identified. Had the same conditions applied in the post-Second World War period, mass emigration from Britain would have continued until the early 1990s. But from the mid-1960s these influences became less powerful as they were increasingly inhibited by immigration policies in the principal destination countries. The long-term decline in emigration is largely accounted for by shifts in policy, especially those that curtailed or abolished the preferences previously extended to settlers from the UK.