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.