This book provides a systematic overview of the prevalence, causes, and stability of left-wing and right-wing extremist attitudes in Germany between 1994 and 2017. It shows that there are many similarities between left-wing and right-wing extremists, both in terms of their ideologies and their individual experiences. Overall, these causes can be traced back to three factors: unmet individual needs (e.g., deprivation or disenchantment with politics), access to ideological narratives that promise simplified solutions to individual problems, and the larger social circumstances of life (e.g., transformation processes, unemployment, or immigration). Although extremist attitudes are relatively rare, they are also shown to be highly stable: once acquired, individuals are difficult to bring back onto the democratic path. This book is the first to systematically compare left-wing and right-wing extremist attitudes, to provide an intensive methodological contribution to the measurability of such attitudes, and to relate their causes and stability.