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 daʻwa islamiyya). But what exactly does »Call to Islam« mean? After a brief historical sketch of different forms of daʻwa, this book provides an overview of various daʻwa theologies of the 20th and 21st centuries as well as of some daʻwa organizations and different daʻwa approaches. Finally, the question is raised about the challenges that daʻwa activities of a conservative or an Islamist Islam pose for liberal and democratic societies.