Conversations about the role and value of Islamic diversity in Indonesia's Islamic public sphere are becoming more frequent and intense. For some Muslims, homogeneity is a precondition for a prosperous and pious community. For others, diversity is a resource that is necessary for creating a just society, and for preserving Indonesia's religious, political and social distinctiveness. Indonesia's regional Islamic traditions are increasingly being cited as reference points in these conversations. Hasan Mustapa (1857-1930) was a scholar, mystic and poet who studied in Mecca for thirteen years before commencing his career as an Islamic official in the Netherlands East Indies. He wrote a number of sufistic treatises on Islamic belief and practice, mostly in the Sundanese language. To the surprise of many, his name and writings are now being more frequently referenced in public discourse. Indonesians are becoming more interested in his work, which they interpret as a characteristically Indonesian mediation of Islamic concepts belonging to the intellectual lineage of figures such as Ibn al-'Arabi (d. 1240) and 'Abd al-Karim Al-Jili (d. 1424). Members of the Sundanese ethnic group of West Java, who currently number around forty million, have also shown renewed interest in his work as a model for nurturing a pro-diversity ethic in the province's unsettled Islamic public sphere. Hasan Mustapa: Ethnicity and Islam in Indonesia is comprised of chapters by Sundanese scholars, alongside the editor's contributions. Some provide introductions to Mustapa's life and work, while others perform a discursive move of increasing importance in contemporary Indonesia: reaching into a regional Islamic past to make authoritative statements about the present. Together, the chapters form a timely addition to the literature on a question of growing importance: what influence should regional traditions have in contemporary Islamic societies?