jats:p The historical significance of Sir Robert Cotton's famous library of manuscripts is considered through the activities of James Ussher, Archbishop of Armagh, who used the library extensively in his work as Protestant theologian and historian. Cotton's library is singled out for particular praise on more than one occasion in Ussher's published works, and it is revealed that Ussher used nearly one hundred Cottonian manuscripts. This use is attested to in his notebooks and correspondence, the surviving records of loans made from the Cottonian Library, and from some of the Cottonian manuscripts themselves. Ussher's manuscript use is analysed with reference to his antiquarian interests as well as to his politico-theological aims for the furtherance of the reformed, established Church in Ireland. </jats:p>