New Testament scholars often claim that the interpretative key to Jesus' pronouncement of the words egô eimi in the Gospel of John lies in the use of this phrase in the Septuagint of Isaiah to render the Hebrew expression 'anî hû'. While previous studies have paid particular attention to the New Testament usage of egô eimi, Catrin H. Williams sets this evidence within a broader framework by offering a detailed analysis of the interpretation of 'anî hû' in biblical and Jewish traditions. She examines the role of 'anî hû' as a succinct expression of God's claim to exclusiveness in the Song of Moses and the poetry of Deutero-Isaiah, and attempts to reconstruct its later interpretative history from the substantial body of evidence preserved in the Aramaic Targumim and several midrashic traditions. Biblical 'anî hû' declarations are cited by rabbinic authorities as proof-texts against a variety of heretical claims, particularly the 'two powers' heresy, but new 'anî hû' formulations, not necessarily confined to divine speeches, are also attested. In the concluding chapters Catrin H. Williams considers the role of 'anî hû' when seeking to interpret Jesus' utterance of the words egô eimi in Synoptic and Johannine traditions.