A pump, for use in low-profile applications e.g. a syringe, comprises a barrel 30 and a piston 20 the piston comprising a plunger 21 to move within the barrel and a sheath 22 to move around an outer surface of the barrel, where the inner surface of the sheath and the outer surface of the barrel have complimentary screw threads so that rotating the piston causes the piston to travel along the barrel. Preferably the plunger is hollow and the piston comprises an engagement section for engaging with a driving element 41 to turn the piston, where the engagement section has an opening into the hollow plunger and a slot for engaging the driving element. A method of pumping fluid and a system comprising a casing, a pump and an actuator are also claimed.
