An injection device includes a housing for receiving a syringe or cartridge for containing a medicament; a rotary driveshaft mounted for rotation relative to the housing; and an elongate coiled torsion spring having respective formations at opposite ends thereof, with a first formation being anchored in a seat on the rotary drive shaft and a second formation being anchored in a seat on a reaction component. In use relative rotation of the rotary driveshaft and the reaction component in one angular direction strains the torsion drive spring, and release of the strained torsion drive spring causes expression of medicament from the syringe or cartridge. The torsion spring is longitudinally resiliently compressible between its end formations whereby during assembly of the injection device, the torsion spring may be longitudinally compressed and then rotated, thereby urging the end formations of the torsion drive spring into engagement with the respective seats.