An injection device comprising: - a medicament cartridge having an a cartridge septum adapted to be pierced by a needle for establishing fluid communication with the cartridge interior and having a slideably arranged piston which is driveable towards the cartridge septum, - a piston driver capable of driving the piston towards the cartridge septum, - a needle unit having a front needle for penetrating the skin of a subject user and a rear needle for piercing the cartridge septum, the front needle and rear needle being adapted for fluid communication, the cartridge and the rear needle being configured for relative movement from a first state where the cartridge septum is sealed to a second state where the cartridge septum is pierced by the rear needle, - a needle shield associated with the needle unit, the needle shield and the front needle being configured for relative movement from a shielded state where the front needle (510; 510') is shielded into an unshielded state where the front needle protrudes from the needle shield, - an activator defining a housing section which at least partly accommodates said cartridge and where the housing section is adapted to be gripped by the hand of a user, the activator being mechanically coupled to the needle unit to transfer a force from the activator to the needle unit when the activator is moved relative to the needle shield so as to cause the front needle and the needle shield to move relative to each other, - a holding mechanism (380,300,370,373,314,310,313) associated with the needle unit and the cartridge for releasably maintaining the needle unit and the cartridge in the first state, the holding mechanism (380,300,370,373,314,310,313) including a releasable retainer (373,314) configured to retain the piston driver relative to the needle unit and configured to be released upon the front needle being shifted from the shielded state to the unshielded state allowing the piston driver to move relative to the needle unit, and - an actuator providing a stored energy source coupled to the piston driver and configured for driving the piston driver upon release of the holding mechanism to cause the cartridge to move relative to the rear needle to enter into the state where the cartridge septum is pierced by the rear needle and to cause the piston driver to move to dispense the medicament through the needle unit, wherein the front needle is manually operable relative to the needle shield by movement of the housing section relative to the needle shield such that when the needle shield is held against an injection site, operation of the front needle causes manual penetration of the front needle into the injection site for subsequent release of the holding mechanism (380,300,370,373,314,310,313), and wherein the holding mechanism (380,300,370,373,314,310,313) is configured for maintaining the rear needle and the cartridge in the first state until the front needle protrudes a predetermined distance from the needle shield and whereupon, when the front needle protrudes farther from the needle shield than said predetermined distance, the holding mechanism (380,300,370,373,314,310,313) is released for shifting the rear needle and the cartridge into its second state. - a medicament cartridge having an a cartridge septum adapted to be pierced by a needle for establishing fluid communication with the cartridge interior and having a slideably arranged piston which is driveable towards the cartridge septum, - a piston driver capable of driving the piston towards the cartridge septum, - a needle unit having a front needle for penetrating the skin of a subject user and a rear needle for piercing the cartridge septum, the front needle and rear needle being adapted for fluid communication, the cartridge and the rear needle being configured for relative movement from a first state where the cartridge septum is sealed to a second state where the cartridge septum is pierced by the rear needle, - a needle shield associated with the needle unit, the needle shield and the front needle being configured for relative movement from a shielded state where the front needle (510; 510') is shielded into an unshielded state where the front needle protrudes from the needle shield, - an activator defining a housing section which at least partly accommodates said cartridge and where the housing section is adapted to be gripped by the hand of a user, the activator being mechanically coupled to the needle unit to transfer a force from the activator to the needle unit when the activator is moved relative to the needle shield so as to cause the front needle and the needle shield to move relative to each other, - a holding mechanism (380,300,370,373,314,310,313) associated with the needle unit and the cartridge for releasably maintaining the needle unit and the cartridge in the first state, the holding mechanism (380,300,370,373,314,310,313) including a releasable retainer (373,314) configured to retain the piston driver relative to the needle unit and configured to be released upon the front needle being shifted from the shielded state to the unshielded state allowing the piston driver to move relative to the needle unit, and - an actuator providing a stored energy source coupled to the piston driver and configured for driving the piston driver upon release of the holding mechanism to cause the cartridge to move relative to the rear needle to enter into the state where the cartridge septum is pierced by the rear needle and to cause the piston driver to move to dispense the medicament through the needle unit,