Political parties maintain local organisations and recruit members mainly to fight elections. For most of the post-war period, however, the dominant view among analysts has been that constituency campaigning in British general elections has little or no effect on election outcomes. This view has been challenged over the last ten years or so. Evidence derived from post-election surveys of constituency election agents following the 1992, 1997 and 2001 general elections is used here to show that the intensity of constituency campaigning significantly affects turnout levels and, for Labour and the Liberal Democrats, levels of party support. There is also some evidence that Conservative campaigning affected constituency variations in the party's performance in 2001. The conclusions reached on the basis of aggregate-level analysis are supported by analysis of individual-level data derived from British Election Study surveys. The effects of campaigning are not large, but they are clear and significant – and sufficient to affect the numbers of seats won by the major parties. In the light of this, parties have good reasons to maintain healthy local organisations.