Provided is a drive unit for an auto-injector having a drive unit housing arranged for docking receipt of a syringe or of a cassette unit comprising a syringe movable from a rest position, in which a needle tip of the syringe is within the drive unit housing to a use position, in which the needle tip protrudes from a needle delivery aperture； and a drive arrangement including one or more elec-trically powered sources of axial drive； a first drive transfer element for advancing the syringe to said use position； and a second drive transfer element for moving a plunger into the barrel of the syringe to eject liquid contents thereof. The drive unit housing is provided with a skin sensor arrangement having an array of plural skin sensor electrodes located about the needle delivery aperture.
