Triggered by the recent Paris attacks the authors make an attempt to answer the question how liberal democracies can react to religious extremists that respond with violence to utterances they consider to be blasphemous. After a brief historical survey of the reactions to blasphemy in penal law and philosophy, the authors compare blasphemy laws and their relation to the freedom of speech in various European countries. Then they analyse the relevant case law of the European Court of Human Rights and the U.S. Supreme Court. In their conclusion the authors defend the position that liberal democracies should be cautious in order to prevent the principles of liberal democracy from being subverted by self-censorship induced by fear of extremism.