A cannula insertion device comprises an elongate frame (4) and a hub (5) mounted to the frame and arranged to be slidably moveable from a first position forwards along a longitudinal axis to a second position relative to the frame and a needle (11) projecting forwards from the hub and carrying a detachable cannula (2) assembly including a cannula. The frame and hub are arranged to allow the hub to be moved forwards from the first position to the second position so as to insert the cannula into a patient and to be drawn backwards to a withdrawn position. The needle is shielded by the frame when the hub is in the first position and also when in the withdrawn position.