The invention provides a closure suitable for use on pipelines and pressure vessels, and which can resist high pressure but which can be opened quickly when the pressure differential across the closure has been reduced to zero. The closure comprises a door (3) which fits into an aperture of a hub (1) in a member (2) to be closed. A number of arcuate locking members or segment blocks (30) disposed around the periphery of the door for locking the door (3) to the hub (1). The locking members (30) are constrained by guide pins (34) to move along guide slots (40) in the door. A drive plate (101) is rotatably mounted on the door (3) and includes a plurality of drive slots (122) which engage with the guide pins (34). Rotation of the drive plate (101) relative to the door (3) causes the locking members (30) to move radially outwardly and circumferentially such that the locking members are moved from an unexpanded unlocked position to an expanded locking position in which the locking members (30) engage both the door (3) and the hub (1) thereby preventing opening of the door. A key member (82) is moved to a position between two adjacent locking members(30) to lock the closure. The closure is unlocked by removing the key member (82) and then rotating the drive plate (101) to move the locking members (30) back to the unexpanded unlocked position.