In this article, I discuss surveys, academic debates, and research on antisemitism among Muslims in Muslim-majority countries and in Europe today. After a review of antisemitism in both contexts, different explanations for its causes are presented. Negative attitudes towards Jews in Muslim countries are the rule, not the exception. An important factor in almost all Muslim countries is the anti-Zionist attitudes and agitations that are mixed with anti-Semitic stereotypes and conspiracy theories. In Europe, antisemitism is more prevalent among Muslims than among non-Muslims and there is a disproportionate involvement of Muslims in anti-Semitic incidents. It turns out that antisemitism among Muslims is manifested in many ways and that it has many causes. One-dimensional explanations are not sufficient. In view of new research, the arguments that antisemitism is primarily a result of Middle East conflict or of discrimination/colonization seem to be outdated. Historically, the interaction of Arab nationalism, Islamist movements, and the collaboration with the Nazis in the middle of the twentieth century played a significant role in ensuring that the discriminatory treatment of Jews by Islam in the Middle Ages did not disappear over the course of establishing nation-states, but was converted to a large extent into antisemitism. Today, Islamist influences and stereotypes that are passed on by media and within social circles are essential factors for the prevalence of anti-Semitic attitudes among Muslims.