Why do parties other than major parties survive or even flourish under plurality rule electoral systems, when according to Duverger's law we should expect them to disappear. Why should rational voters support third parties, even though their chances of being successful are often low . Using an institutional public choice approach, this study analyses third party voting as one amongst a continuum of choices faced by electors who pay attention both to the ideological proximity of parties, and to their perceived efficacy measured against a community-wide level of minimum efficacy. The approach is applied in detailed case study chapters examining four different third parties. Two of the cases cover long-established and relatively successful third parties - the British Liberal Democrats; the Canadian NDP. The other two cases cover shorter-lived third parties - the New Zealand Social Credit; and the UNP in South Korea. In each case the study examines the party's specific history and dynamics, looks at the social base of its support and its ideological positioning, explores the party's perceived efficacy, and analyses the articulation of the third party's strategy. Two key themes emerge. First, plurality rule electoral systems impose severe constraints on third parties, but also create niches (such as one-party safe seats or regions, or unoccupied ideological space) within which a long-term third party can become established, flourish and develop strategies to partially overcome its lower perceived efficacy. Second, third party voting under plurality rule is not an isolated behaviour, but part of an integrated spectrum of choices (encompassing abstention, protest voting, tactical voting, and positive party support) which citizens make. People respond both to the ideological positioning and to the perceived efficacy of the competing political parties, within a specific voting context and using a collectively defined sense of what constitutes efficacious political behaviour.