A medicament delivery device for the administration of one or more drug agents, wherein the device comprises: a retainer for holding a medicament cartridge, the retainer being moveable between an open and closed position; a position sensor for generating a position output indicative of whether the retainer is in an open or closed position; a cartridge sensor for generating a cartridge output indicative of whether the retainer is holding a medicament cartridge; a piston rod and drive train; a controller for controlling a predetermined operation of the device in dependence on an output of the position sensor and on an output of the cartridge sensor; a dispense interface for providing fluidic communication between an outlet and a medicament cartridge retainer; an interface sensor for generating an output indicative of whether the dispense interface is attached to or detached from the device; a latch movable under control of the controller between a latched position, for holding the retainer in said closed position, and an unlatched position, for allowing opening of the retainer to the open position; and a cartridge door button; the controller being operative for controlling a predetermined operation of the device in dependence on an output of the interface sensor, wherein the predetermined operation relates to a cartridge change cycle of the device upon detachment of the dispense interface, wherein, in response to receiving the output from the interface sensor indicating that the dispense interface is detached from the device, the controller is configured to initiate the cartridge change cycle, wherein initiating the cartridge change cycle comprises: activating said cartridge door button; and in response to detecting that the activated cartridge door button has been actuated by a user, retracting the piston rod from the medicament cartridge and moving the latch from the latched position to the unlatched position. a retainer for holding a medicament cartridge, the retainer being moveable between an open and closed position; a position sensor for generating a position output indicative of whether the retainer is in an open or closed position; a cartridge sensor for generating a cartridge output indicative of whether the retainer is holding a medicament cartridge; a piston rod and drive train; a controller for controlling a predetermined operation of the device in dependence on an output of the position sensor and on an output of the cartridge sensor; a dispense interface for providing fluidic communication between an outlet and a medicament cartridge retainer; an interface sensor for generating an output indicative of whether the dispense interface is attached to or detached from the device; a latch movable under control of the controller between a latched position, for holding the retainer in said closed position, and an unlatched position, for allowing opening of the retainer to the open position; and a cartridge door button; activating said cartridge door button; and in response to detecting that the activated cartridge door button has been actuated by a user, retracting the piston rod from the medicament cartridge and moving the latch from the latched position to the unlatched position.