A fluid flow control system for a well, comprising a choke adapted for downhole use in an oil or gas well to control the flow of fluids in a reservoir, the choke having an axis, and comprising: a conduit with a bore; a first opening (Ap) allowing fluid communication between the outside of the conduit and the choke; a second opening allowing fluid communication between the choke and the bore of the conduit; a flow path connecting the first (Ap) and second openings; a flow restrictor that is selectively adjustable to change a cross sectional area of the flow path through the choke, the flow restrictor comprising first and second choke members (10,20), each incorporating a portion of the flow path; wherein at least one of the first and second choke members (10,20) is rotatably mounted to rotate relative to the other to move the flow path portion on at least one of the first and second choke members (10,20) relative to the flow path portion on the other, to change a cross-sectional area of the flow path; wherein each choke member (10,20) has a mating face that engages a cooperating mating face of the other choke member (10,20), and wherein each mating face has an aperture (12, 22) that moves relative to the aperture (12,22) on the mating face of the other choke member as the at least one choke member rotates relative to the other choke member, and wherein the first and second choke members (10,20) are axially spaced from one another; and a shifting tool (50,150) configured to drive at least one of the choke members in relative rotation to the other; characterised in that the shifting tool (50,150) is adapted to be deployed on a line or string of tools passing through the bore of the choke. a conduit with a bore; a first opening (Ap) allowing fluid communication between the outside of the conduit and the choke; a second opening allowing fluid communication between the choke and the bore of the conduit; a flow path connecting the first (Ap) and second openings; a flow restrictor that is selectively adjustable to change a cross sectional area of the flow path through the choke, the flow restrictor comprising first and second choke members (10,20), each incorporating a portion of the flow path; wherein at least one of the first and second choke members (10,20) is rotatably mounted to rotate relative to the other to move the flow path portion on at least one of the first and second choke members (10,20) relative to the flow path portion on the other, to change a cross-sectional area of the flow path; wherein each choke member (10,20) has a mating face that engages a cooperating mating face of the other choke member (10,20), and wherein each mating face has an aperture (12, 22) that moves relative to the aperture (12,22) on the mating face of the other choke member as the at least one choke member rotates relative to the other choke member, and wherein the first and second choke members (10,20) are axially spaced from one another; and a shifting tool (50,150) configured to drive at least one of the choke members in relative rotation to the other; characterised in that the shifting tool (50,150) is adapted to be deployed on a line or string of tools passing through the bore of the choke.