A medicament delivery device for delivering a dose of medicament into an injection site comprises an expandable reservoir (130) for receiving the medicament, a holder (102) for receiving the reservoir (130), attachment means (104, 110) for securing the holder (102) to the injection site, a cannula (174) moveable from a stowed position in which the cannula (174) does not project from the device (100) to a deployed position in which the cannula (174) provides fluid connection between the reservoir (130) and the injection site, and an elastic element (106). In use, the elastic element (106) is held under strain by the medicament in the reservoir (130) and delivery of the medicament through the cannula (174) is driven by relaxation of the elastic element (106) when the cannula (174) is in the deployed position. The device is preferably in the form of an arm band or a patch.
