Nasir al-Din Tusi, the renowned Shi'i scholar of the 13th century, produced a wide range of writings in different fields of learning under Ismaili patronage and later under the Mongols. His work brought him great acclaim and, as a result of his changing affiliations in the course of a brilliant scholastic career, he is claimed by both Twelver Shi'i and Ismaili traditions. Contemplation and Action is Tusi's spiritual autobiography in which he recounts details of his early education, his search for knowledge and his eventual conversion to the Ismaili faith. Translated for the first time into English, it is published here with the full Persian text and a substantial introductory essay on Tusi's life and thought.