Today Islam is numerically the second largest religion in the world. Its message is aimed generally at all people and has been addressed to Muslims and non-Muslims alike since the beginning of Islam through the »Call to Islam« (Arabic da wa islamiyya). But what exactly does »Call to Islam« mean? After a brief historical sketch of different forms of Da wa, this book provides an overview of various Da wa theologies of the 20th and 21st centuries as well as of some Da wa organizations and different Da wa approaches. Finally, the question is raised about the challenges that Da wa activities of a conservative or an Islamist Islam pose for liberal and democratic societies.