Described is a drive mechanism for a drug infusion pump. In one embodiment, an in-line drive mechanism is provided that includes a motor operatively coupled to a lead screw, which is configured to engage a piston. The piston includes a cavity to receive the motor and the lead screw such that the lead screw and at least a portion of the motor are substantially contained within the piston cavity when the piston is in a retracted position. In one embodiment, the motor rotates a drive shaft is configured to turn a mechanism comprising a piston, a lead screw, and a housing that cooperate to extend telescopically to drive a plunger into a medicament cartridge, thereby expelling fluid.