This is the first comprehensive critical edition of a text which was a fundamental influence on Jewish thought in the medieval period and has continued to fascinate scholars and students of Judaism to the present day. It was initially understood to be a philosophical text which had descended by oral tradition from Abraham himself. It purports to tell us how God created the world using the ten sefiroth (the Spirit of the living God, air, water and fire, and the twenty-two letters of the Hebrew Alphabet). With its English translation of the three earliest recensions and its commentary on the variant early texts of the work, this book is essential reading for anyone interested in the growth and emergence of the Jewish mystical movement. There are four appendices setting out what parts of the text are attested in each of the manuscripts and in what order, a hypothetical reconstructed text and the text of the tenth century Vatican scroll of Sefer Yesira with the probable added ma terial underlined.