This thesis assesses the existence of a populist zeitgeist in Europe. Even though populism and radical right-wing ideology often coincide empirically,I argue for treating both concepts as distinct features that ought to be studied using different theoretical lenses and methods. This becomes particularly crucial, when we study their diffusion and contagion effects on mainstream parties. Hence, my dissertation aims at disentangling the effect of radical right ideology as a fully-fledged thick ideology and populism as a thin ideology attached to it. The thesis is structured in four papers. Chapter 2 consists of an automated systematic review of populism research applying text-as-data-methods to the abstracts of all political sciences articles published between 2004 and 2018. I show that populism research is divided by geographical foci, methods, and conceptions of populism. This stems from a common overstatement of populism’s significance, which comes at the expense of its host ideologies, thus confounding the effects of these thick and thin ideologies. Based on this finding, I argue to study the contagion effect of populism and radical-right ideology thoroughly and separately. In order to do so, my third chapter develops a novel automated approach to measure populist discourse, which is a prerequisite to study its spreading in political discourse. I argue that established theoretical assumptions, i.e. presenting the people as morally superior and the elite as evil, are a valuable means to identify populist discourse. My two-step dictionary approach allows to detect references to both groups and to identify whether they are framed in a moralizing way. Subsequently, the last two chapters of my thesis assess a possible contagion of mainstream parties with populist rhetoric and radical right programmatic appeals in a comparative perspective. Serving as an example on how to study radical right programmatic contagion, the chapter 4 focuses on the impact of the refugee crisis and radical right party pressure on party competition regarding immigration in Austria, Germany, and Switzerland. Using 120,000 party press releases issued between 2013 and 2017 and text-as-data methods, we calculate monthly measures of salience and positions. We show how the 2015 crisis and the success of radical right parties interact by studying how mainstream parties react to the behaviour of the radical right in terms of salience and positional change. Finally, chapter 5 studies the diffusion of populism in the EP and assesses whether mainstream parties adapt a populist discourse. Theoretically, I argue that scholars ought to study populist contagion in terms of what populism is: an ideational, thin feature of parties, rather than a fully-fledged programmatic orientation. Thus, the study uses my measurement of populist discourse in order to show its development over time and across actors, particularly assessing whether there are time trends and mutual influences between parties.