A drug delivery device, comprising: a body (104, 116) having a reservoir (160, 164, 176) disposed therein for containing a medicament, the reservoir including a flexible wall; a plunger (144, 508, 364) movable within the body for causing the medicament to be expelled from the reservoir, the plunger having a contact surface that is not affixed to said flexible reservoir wall; a spring or a different type of stored energy device biasing the plunger in a pre-activated position toward the reservoir; and means (200, 204, 218, 504, 516, 520, 564, 568, 572, 582, 584) for selectively maintaining the plunger in the pre-activated position with respect to the reservoir and, upon releasing the plunger from the pre-activated position, for guiding the plunger to move under the force of the spring or a different type of stored energy device such that the contact surface of the plunger contacts the flexible reservoir wall to pressurize the reservoir for delivery of the medicament to a patient characterized in that: the device further comprises a rotor rotatably disposed about the housing and being rotatable between a pre-activated position and an activated position; wherein during rotation from the pre-activated position to the activated position, a tab engaging surface of the rotor rotates the plunger tab from engagement with the tab engaging portion of the housing to alignment with the recessed channel, thereby permitting the plunger (144, 508, 564) to move under the force of the spring or a different type of stored energy device to pressurize the reservoir for delivery of the medicament to a patient wherein the means for selectively maintaining and guiding the plunger comprises a housing disposed within the body (104, 116), the plunger and spring or a different type of stored energy device being disposed within the housing; wherein the plunger includes at least one tab extending from an outer edge thereof; wherein the housing includes at least one opening with a tab engaging portion for selectively engaging the plunger tab and at least one recessed channel, for guiding the at least one plunger tab during plunger movement under the force of the spring or a different type of stored energy device to pressurize the reservoir.; and wherein the at least one opening in the housing comprises an L-shaped opening having a vertical portion substantially aligned with the recessed channel, and a horizontal portion including the tab engaging portion. a body (104, 116) having a reservoir (160, 164, 176) disposed therein for containing a medicament, the reservoir including a flexible wall; a plunger (144, 508, 364) movable within the body for causing the medicament to be expelled from the reservoir, the plunger having a contact surface that is not affixed to said flexible reservoir wall; a spring or a different type of stored energy device biasing the plunger in a pre-activated position toward the reservoir; and means (200, 204, 218, 504, 516, 520, 564, 568, 572, 582, 584) for selectively maintaining the plunger in the pre-activated position with respect to the reservoir and, upon releasing the plunger from the pre-activated position, for guiding the plunger to move under the force of the spring or a different type of stored energy device such that the contact surface of the plunger contacts the flexible reservoir wall to pressurize the reservoir for delivery of the medicament to a patient characterized in that: the device further comprises a rotor rotatably disposed about the housing and being rotatable between a pre-activated position and an activated position; wherein during rotation from the pre-activated position to the activated position, a tab engaging surface of the rotor rotates the plunger tab from engagement with the tab engaging portion of the housing to alignment with the recessed channel, thereby permitting the plunger (144, 508, 564) to move under the force of the spring or a different type of stored energy device to pressurize the reservoir for delivery of the medicament to a patient wherein the means for selectively maintaining and guiding the plunger comprises a housing disposed within the body (104, 116), the plunger and spring or a different type of stored energy device being disposed within the housing; wherein the plunger includes at least one tab extending from an outer edge thereof; wherein the housing includes at least one opening with a tab engaging portion for selectively engaging the plunger tab and at least one recessed channel, for guiding the at least one plunger tab during plunger movement under the force of the spring or a different type of stored energy device to pressurize the reservoir.; and wherein the at least one opening in the housing comprises an L-shaped opening having a vertical portion substantially aligned with the recessed channel, and a horizontal portion including the tab engaging portion.