Recent experimental work has shown that adhesively bonded inserts can significantly increase the fatigue life of fastener holes. This paper concentrates on developing an understanding of the mechanisms which give rise to the observed increase in life. Cracked as well as uncracked fastener holes are considered. It is shown that both the stress concentration factors and the stress intensity factors are significantly reduced by the use of either bonded rivets or bonded sleeves. Furthermore the stress intensity factor, for a cracked hole repaired by a bonded insert, can be obtained by analogy with readily available solutions. The ability to repair cracked structures using this method is then confirmed by a series of experimental tests.