A medicament delivery device comprises a housing, a telescopic piston rod assembly for driving a bung of a medicament container, and a drive mechanism for the telescopic piston rod assembly. The telescopic piston rod assembly comprises a plunger for driving a bung, a key for allowing predominantly axial movement between the plunger and the housing and an input drive telescopically coupled to the plunger and supported for rotational and axial movement relative the housing. The drive mechanism additionally comprises a transfer drive that operates to rotate the input drive. As the input drive rotates, it moves axially relative to the housing, expanding telescopically relative to the plunger.