Differentiating delusional disorder from extreme religious or political beliefs is a diagnostic challenge for mental health professionals. When the radicalization of these extreme beliefs leads to violent expression, this differential becomes important to courts as well (e.g., questions of insanity or competence to stand trial). These challenges are paradoxical, considering the long history of the syndrome in the scholarly literature and the straightforward definitions and associated diagnostic criteria. More specifically, delusions are fixed beliefs that are not amenable to change in light of conflicting evidence; delusional disorder involves the presence of delusions in the absence of other significant psychopathology (see DSM-5-TR). When the fixed false beliefs are thoroughly idiosyncratic (e.g., a spouse is conspiring with various Fed-Ex workers to deliver toxic packages to the home), there is little need for additional diagnostic guidance. More problematic is the presentation of fixed beliefs having some overlap in content or theme with extreme religious or political beliefs held by others in a culture.