A catheter assembly comprising: a hub assembly comprising a hub and a catheter attached to the hub; and a needle retracting device releasably attached to hub assembly which is arranged to project a needle through the catheter to allow catheterization and, in response to user actuation, to retract the needle and to be detached or to allow detachment from the hub assembly, the needle retaining device comprising: a chassis extending along an axis from a base; and a slider carrying the needle supported on the chassis, arranged to be moveable away from the base along the axis from a first position in which the needle projects through the catheter to a second position in which the needle is withdrawn from the catheter and is shielded by the chassis; wherein the chassis and hub comprise movable locking members and cooperating locking surfaces for engaging the locking members and wherein the slider comprises retaining surfaces for limiting movement of the locking members, wherein the catheter assembly is configured such that: when the slider is in the first position, the locking members are discouraged or prevented from disengaging from the locking surfaces by the retaining surfaces; and when the slider is in the second position, the locking members are free to disengage from the locking surfaces and so allow the needle retracting device to disengage from the hub assembly; wherein the chassis comprises: a tubular body extending from the base portion to a distal end along the axis; and the slider comprises: a tubular body disposed outside the tubular body of the chassis; a hub carrying the needle; and characterised by the slider comprising member(s) connecting the hub to the tubular body of the slider, wherein an inner wall of the tubular body of the slider provides the retaining surfaces. a hub assembly comprising a hub and a catheter attached to the hub; and a needle retracting device releasably attached to hub assembly which is arranged to project a needle through the catheter to allow catheterization and, in response to user actuation, to retract the needle and to be detached or to allow detachment from the hub assembly, the needle retaining device comprising: a chassis extending along an axis from a base; and a slider carrying the needle supported on the chassis, arranged to be moveable away from the base along the axis from a first position in which the needle projects through the catheter to a second position in which the needle is withdrawn from the catheter and is shielded by the chassis; a chassis extending along an axis from a base; and a slider carrying the needle supported on the chassis, arranged to be moveable away from the base along the axis from a first position in which the needle projects through the catheter to a second position in which the needle is withdrawn from the catheter and is shielded by the chassis; when the slider is in the first position, the locking members are discouraged or prevented from disengaging from the locking surfaces by the retaining surfaces; and when the slider is in the second position, the locking members are free to disengage from the locking surfaces and so allow the needle retracting device to disengage from the hub assembly; a tubular body extending from the base portion to a distal end along the axis; a tubular body disposed outside the tubular body of the chassis; a hub carrying the needle; and characterised by the slider comprising member(s) connecting the hub to the tubular body of the slider,