A retention system for retaining a stent graft ( 13 ) onto a deployment device ( 1 ) in which the stent graft has an exposed stent ( 11 ) at at least one end. The deployment device has a capsule ( 7 ) to receive the exposed stent during deployment and an arrangement to move the capsule to release the exposed stent when required. A release wire ( 17 ) associated with the deployment device engages a portion ( 19 ) of the exposed stent within the capsule to retain the exposed stent in the capsule. At least one retention loop ( 25 ) on the stent graft is connected around the release wire to prevent removal of the capsule from the exposed stent until the release wire has been removed from its engagement with the exposed stent within the capsule and the retention loop is released.