The invention relates to a valve (4) for a flexible packaging (1), comprising a valve housing (11) to be connected to an aerating and/ or venting opening (6) of the packaging and having a first opening (22) and an opposite second opening (35), and a flexible membrane (24) movable between a position which closes the first opening and a position which leaves the first opening clear. The valve is provided with guide means (28) for guiding the movement of the membrane, wherein there is space (40) in the housing adjacently of the membrane for the purpose of forming a passage between the first and second opening when the membrane takes up-its open position. The invention also relates to a flexible packaging with an aerating and/or venting opening and a valve arranged therein. The invention further relates to an assembly of an aerating and/or venting device and such a valve. The invention also relates to a method for evacuating a flexible packaging via a venting opening while making use of such a valve and a venting device connected thereto. The invention finally relates to a method for flushing a flexible packaging with gas via an aerating and/or venting opening by making use of such a valve and an aerating and/or venting device connected thereto.