By-elections raise fundamental questions, such as: why do third parties manage unexpected breakthroughs in such contests? Why does the government of the day consistently lose support through the `mid-term blues'? Are by-elections essentially idiosyncratic contests reflecting the strengthsand weaknesses of individual candidates in particular constituencies? Or can a series of by-election results provide an accurate indication of party popularity in subsequent general elections? Pippa Norris addresses these questions through her analysis of post-war trends in party support. She covers changes in campaigns, contrasting the stable by-elections of the post-war decade with the more volatile ones characteristic of today. She then explores systematic trends in the light oftheories of partisan dealignment and retrospective voting, analysing the influence of campaign-specific factors, notably the role of candidates, party organizations, the media, and opinion polls. To set Britain in a comparative context, she also surveys trends in by-elections in Canada andAustralia. Finally there is an essential reference section listing changes in party support in almost four hundred British by-elections since 1945.