The left-right ideological positions of political parties play a central role in theorizing about many different aspects of democratic processes. Unfortunately, scholars are hindered in their ability to test existing theories by the limited availability of data that is comparable over time and across countries. This paper describes a simple 'vanilla' method for using manifestos data to estimate party leftright positions. It then tests this method and four existing ones by regressing a variety of accepted survey-based measures of left-right party positions on the estimates of party positions generated by the various techniques. Finally, analysis of the residuals from these regressions identifies the extent to which there are systematic sources of errors in using manifestos data to estimate party left-right positions. The vanilla method consistently produces the best estimates of party positions, and these estimates are quite good (less than one point, on average, from the estimates of other accepted approaches). Manifestos data, however, tend to locate extreme parties closer to the ideological center than do other survey-based approaches. he left-right ideological positions of political parties play a central role in theorizing many different aspects of democratic processes. Scholars employ the left-right positions of parties in theoretical arguments and empirical tests on such topics as macro-economic policymaking, legislative institutional choice, electoral competition, voting behavior, political representation, and cabinet stability. Thus, evaluation of a wide range of theoretical arguments in comparative politics requires reliable measures of the left-right positions of political parties. A practical problem facing scholars, however, is the limited availability of appropriate measures. There are two common sources of data on leftright party locations. One consists of expert surveys on specific party systems, where experts are asked to provide estimates of party left-right positions in these systems. Castles and Mair (1984) published the first such survey in a large number of countries, and results from a similar, subsequent survey are presented in Huber and Inglehart (1995).' Although these surveys are widely used to measure party positions, they suffer important limitations: they have been administered infrequently, in different formats, and only over the last fifteen years. A second source of data is the left-right positions of party supporters as reported in mass surveys such as the Eurobarometer or the World Values Survey. These surveys have been administered more frequently, but only over the last two decades and for a limited number of countries. Thus, scholars are frequently forced to limit their