An auto-injector for hypodermic delivery of medication, comprising: a housing; a cartridge holder positioned within the housing and configured to receive a medication cartridge, the cartridge holder being moveable between a first position and a second position proximal from the first position, wherein, in the first position, the cartridge holder is prevented from moving in a proximal direction; a moveable plunger rod positioned within the housing and biased to move in the proximal direction by a plunger spring; a plunger release latch positioned within the housing to releasably retain the plunger rod in a locked position; a hollow injection needle having a longitudinal cavity through which medication can pass from the medication cartridge; and a needle shield movable relative to the cartridge holder between an extended position enclosing the needle and a retracted position in which at least an end of the needle is not enclosed by the needle shield; a first subassembly including the needle, the cartridge holder, the needle shield, and a shell at least partially encompassing the cartridge holder and the needle shield is releasably coupled to a second subassembly including the housing, the moveable plunger rod, and the release latch, wherein releasable engagement of the shell with the cartridge holder prevents the cartridge holder from moving in the proximal direction when the cartridge holder is in the first position, distal movement of the needle shield both releases the cartridge holder from the first position and releases the latch to allow the plunger rod to move in the proximal direction by means of the plunger spring and urge the cartridge holder from the first position to the second position, and movement of the needle shield from the retracted position to the extended position will automatically release the first subassembly from the second subassembly. a housing; a cartridge holder positioned within the housing and configured to receive a medication cartridge, the cartridge holder being moveable between a first position and a second position proximal from the first position, wherein, in the first position, the cartridge holder is prevented from moving in a proximal direction; a moveable plunger rod positioned within the housing and biased to move in the proximal direction by a plunger spring; a plunger release latch positioned within the housing to releasably retain the plunger rod in a locked position; a hollow injection needle having a longitudinal cavity through which medication can pass from the medication cartridge; and a needle shield movable relative to the cartridge holder between an extended position enclosing the needle and a retracted position in which at least an end of the needle is not enclosed by the needle shield; a first subassembly including the needle, the cartridge holder, the needle shield, and a shell at least partially encompassing the cartridge holder and the needle shield is releasably coupled to a second subassembly including the housing, the moveable plunger rod, and the release latch, wherein releasable engagement of the shell with the cartridge holder prevents the cartridge holder from moving in the proximal direction when the cartridge holder is in the first position, distal movement of the needle shield both releases the cartridge holder from the first position and releases the latch to allow the plunger rod to move in the proximal direction by means of the plunger spring and urge the cartridge holder from the first position to the second position, and movement of the needle shield from the retracted position to the extended position will automatically release the first subassembly from the second subassembly.