This paper introduces three new scales to measure left- and right-wing radical as well as general extremist attitudes that can be applied across Western European countries. We therefore propose a thorough conceptualization of extremist attitudes that consists of two dimensions: general extremism, by which we understand attitudes that oppose the constitutional democratic state, and another dimension that differentiates between right- and leftwing radicalism by which we understand people who take far-reaching but often one-sided positions on political issues (e.g., on nationalism or anti-imperialism) by advocating fundamental socio-political change. Based on data from Germany, Great Britain, and the Netherlands (n = 6,201) we created short indices for general extremism and left- and right-wing radicalism. We check for convergence validity by assessing the psychometric properties of the extracted indices, i.e. their internal coherence and the degree to which a scale is able to distinguish strongly extremist and non-extremist individuals. Finally, we correlate the scales with various constructs that are likely related to extremist attitudes in order to assur external or construct validity. The results indicate that the three scales are highly valid and applicable across three Western European countries. Overall, we find that about two to four percent of citizens in each country hold left-wing or right-wing extremist attitudes.