In the 1840s the Finnish Orientalist Georg August Wallin travelled in the Middle East, where he collected material on Arabic dialects. In order to make contact with the local inhabitants he assumed a Muslim identity and disguised himself as the physician 'Abd al-Wali from Central Asia. His unique material is preserved in diaries and letters which he wrote in his native language, Swedish. By his contemporaries he was considered an eminent scholar, but he died shortly after his seven-year journey, and was therefore able to publish only a fraction of his material. Wallin's life and travels in Egypt, the Arabian Peninsula and Persia are described here by four scholars of Middle Eastern culture and history. They also highlight Wallin's importance as an ethnographic observer and linguistic researcher.