An injection device comprising an outer housing adapted to receive: a syringe for holding a volume of a medicament, said syringe comprising a barrel, a needle at one end of the barrel, and a plunger, axially moveable within the barrel, the needle and barrel being such that at least part of the needle is axially moveable in and out of said outer housing but is biased to be normally wholly inside said housing; the injection device further comprising: an inner housing intermediate the outer housing and at least part of the barrel and/or plunger; an energy source in communication with the inner housing; and blocking means capable of preventing said inner housing moving axially under the influence of said energy source; the device being moveable between two positions, namely a first position in which the inner housing is prevented from moving axially forward by engagement with said blocking means; and a second position in which the inner housing is disengaged from said blocking means and is capable of acting on the syringe under the influence of said energy source to move said at least part of the needle out of said outer housing; characterised in that said device is moveable from said first position to said second position by axial movement of said outer housing relative to said inner housing and actuation of a trigger. a syringe for holding a volume of a medicament, said syringe comprising a barrel, a needle at one end of the barrel, and a plunger, axially moveable within the barrel, the needle and barrel being such that at least part of the needle is axially moveable in and out of said outer housing but is biased to be normally wholly inside said housing; the injection device further comprising: an inner housing intermediate the outer housing and at least part of the barrel and/or plunger; an energy source in communication with the inner housing; and blocking means capable of preventing said inner housing moving axially under the influence of said energy source; the device being moveable between two positions, namely a first position in which the inner housing is prevented from moving axially forward by engagement with said blocking means; and a second position in which the inner housing is disengaged from said blocking means and is capable of acting on the syringe under the influence of said energy source to move said at least part of the needle out of said outer housing; characterised in that said device is moveable from said first position to said second position by axial movement of said outer housing relative to said inner housing and actuation of a trigger. an inner housing intermediate the outer housing and at least part of the barrel and/or plunger; an energy source in communication with the inner housing; and blocking means capable of preventing said inner housing moving axially under the influence of said energy source; the device being moveable between two positions, namely a first position in which the inner housing is prevented from moving axially forward by engagement with said blocking means; and a second position in which the inner housing is disengaged from said blocking means and is capable of acting on the syringe under the influence of said energy source to move said at least part of the needle out of said outer housing; characterised in that said device is moveable from said first position to said second position by axial movement of said outer housing relative to said inner housing and actuation of a trigger.