Various scholars have recently argued that niche parties are to be distinguished from mainstream parties, in particular because the two party types differ in their programmes, behaviour and strategies. However, so far there has been no attempt to provide a concise, measureable definition of the niche-party concept. In this article I argue that niche parties are best defined as parties that compete primarily on a small number of non-economic issues. The occurrence of niche parties is then operationalized and measured using issue salience information provided by expert surveys and manifesto data. After comparing the findings with existing definitions, the main characteristics of the niche parties identified are examined in a final step.