A bi-directional gate valve comprising: (i) a valve body having a valve bore extending therethrough along an axis; (ii) a valve cavity surrounding the valve bore; (iii) a gate member operable within the valve body to open and close the valve bore; (iv) opposed gate sealing faces (16, 18) provided on the gate member; (v) an annular recess (20, 22) provided around the perimeter of the valve bore proximate each of the opposed gate sealing faces of the gate member; characterised in that the valve further comprises (vi) an annular seat member mounted within each recess comprising of an outer fixed seat portion (24b, 26b) and an inner moveable seat portion (24a, 26a); (vii) an annular seat sealing face provided on each inner moveable seat portion (24a, 26a) for sealing against a corresponding gate sealing face; (viii) end walls (32a-f) provided on the valve body remote from the opposed gate sealing faces of the gate member and extending between the valve cavity and the valve bore; (ix) at least one annular sealing member for selectively sealing at two different predetermined annular locations between the end wall proximate one recess and an opposing wall of its corresponding inner moveable seat portion; and (x) at least one annular sealing member for selectively sealing at two different predetermined annular locations between the end wall proximate the other recess and an opposing wall of its corresponding inner moveable seat portion; wherein the seat sealing face of each inner moveable seat portion (24a, 26a) protrudes from a surrounding transverse wall and has an annular sealing contact area (A 3) which is less than an annular area on the corresponding opposing wall which is defined between said two different predetermined annular sealing locations; and wherein the annular sealing contact area (A 3) of the seat sealing face of each inner moveable seat portion (24a, 26a) is located at an intermediate transverse position between the two different predetermined annular sealing locations of its at least one annular sealing member. (i) a valve body having a valve bore extending therethrough along an axis; (ii) a valve cavity surrounding the valve bore; (iii) a gate member operable within the valve body to open and close the valve bore; (iv) opposed gate sealing faces (16, 18) provided on the gate member; (v) an annular recess (20, 22) provided around the perimeter of the valve bore proximate each of the opposed gate sealing faces of the gate member; characterised in that the valve further comprises (vi) an annular seat member mounted within each recess comprising of an outer fixed seat portion (24b, 26b) and an inner moveable seat portion (24a, 26a); (vii) an annular seat sealing face provided on each inner moveable seat portion (24a, 26a) for sealing against a corresponding gate sealing face; (viii) end walls (32a-f) provided on the valve body remote from the opposed gate sealing faces of the gate member and extending between the valve cavity and the valve bore; (ix) at least one annular sealing member for selectively sealing at two different predetermined annular locations between the end wall proximate one recess and an opposing wall of its corresponding inner moveable seat portion; and (x) at least one annular sealing member for selectively sealing at two different predetermined annular locations between the end wall proximate the other recess and an opposing wall of its corresponding inner moveable seat portion;