An implantable substance delivery device comprising: a removable portion and an implantable portion (100, 348, 352), the removable portion comprising an elongate shaft having a lumen and a distal end and the implantable portion (100, 348, 352) being detachably attached to the removable portion; said implantable portion (100, 348, 352) comprising (a) a substance delivery reservoir that is in communication with the lumen of the removable portion such that a therapeutic or diagnostic substance, or a component thereof, may be introduced through the lumen into the reservoir, (b) at least one substance release location through which substance that has been introduced into the reservoir will escape from the reservoir, said reservoir being initially disposed in a first configuration and transitionable to a second configuration by introduction of the substance, or component thereof, into the reservoir; said implantable portion (100, 348, 352) being detachable from the removable portion such that the removable portion may be removed from the subject's body leaving the implantable portion within the subject's body, characterized in that at least a part of the implantable portion (100, 348, 352) is implantable within a paranasal sinus and said implantable portion further comprises an anchoring or retention element (350, 354), said anchoring or retention element comprising flexible projections having a first configuration that permits transnasal insertion through an opening of the paranasal sinus by pushing the device in a distal direction and a second configuration in which the flexible projections extend in a radially outward direction thereby increasing the profile of the device and preventing slipping of the device out of the opening in the paranasal sinus but allow removal of the device by pulling on the device. a removable portion and an implantable portion (100, 348, 352), the removable portion comprising an elongate shaft having a lumen and a distal end and the implantable portion (100, 348, 352) being detachably attached to the removable portion; said implantable portion (100, 348, 352) comprising (a) a substance delivery reservoir that is in communication with the lumen of the removable portion such that a therapeutic or diagnostic substance, or a component thereof, may be introduced through the lumen into the reservoir, (b) at least one substance release location through which substance that has been introduced into the reservoir will escape from the reservoir, said reservoir being initially disposed in a first configuration and transitionable to a second configuration by introduction of the substance, or component thereof, into the reservoir; said implantable portion (100, 348, 352) being detachable from the removable portion such that the removable portion may be removed from the subject's body leaving the implantable portion within the subject's body, characterized in that at least a part of the implantable portion (100, 348, 352) is implantable within a paranasal sinus and said implantable portion further comprises an anchoring or retention element (350, 354), said anchoring or retention element comprising flexible projections having a first configuration that permits transnasal insertion through an opening of the paranasal sinus by pushing the device in a distal direction and a second configuration in which the flexible projections extend in a radially outward direction thereby increasing the profile of the device and preventing slipping of the device out of the opening in the paranasal sinus but allow removal of the device by pulling on the device.