When Otto Habsburg, son of the last emperor of the Austria-Hungarian Empire Karl I, died in 2011, among those who prayed at the funeral in Vienna's Stephansdom was Mustafa Efendi Ceri, the Reis ul-ulema of Bosnia and Herzegovina. This was to honour the long-lasting relationship between the bygone empire of the Catholic Habsburgs with European Muslims. The cornerstone of this association was the 1912 Islam Act (Islamgesetz) of the Habsburgs, which made Austria-Hungary the first Catholic European state to confer official status to Islam. This book explores the legacy of this act and the ways in which it continues to impact the legal frameworks and political structures governing Islam and Muslim communities in the successor states of Austria-Hungary. It discusses the unique coexistence in Central Europe of centuries-old, 'indigenous' European Muslims with recently settled Muslim immigrants, and the trajectory of their interactions with the state. This volume is therefore not only crucial to the debate about European Islam but also to the question of the legal and political framework of Islamic religious communities in a secular Europe.