A crossflow filter device for filtering a pressurised feed liquid; wherein the crossflow filter device comprises: a filter membrane ; a flow channel for the pressurised feed liquid which extends in a path over a retentate surface of the membrane such that the direction of flow in the channel is tangential to the retentate surface, and a filtrate derived from the feed liquid passes through the membrane leaving retentate liquid in the flow channel; and a collection chamber for the filtrate formed on an opposite, filtrate surface of the membrane; wherein the crossflow filter device further comprises a sealed housing having a retentate side and a filtrate side which enclose therebetween the flow channel , the filter membrane and the collection chamber; and wherein the crossflow filter device further comprises flow channel guide walls provided at an inner surface of the retentate side of the housing, the guide walls being configured such that the path for the flow channel shuttles back and forth over the retentate surface of the membrane , characterised in that : the flow channel guide walls , which are formed of resiliently deformable elastomer, deform and sealingly engage with the filter membrane to form a fluid tight seal with the filter membrane and thereby define the path of the flow channel over the retentate surface of the membrane, and eliminate short-circuiting of the flow channel by the feed liquid; and the inner surface of the retentate side of the housing includes a recess corresponding to the line of the guide walls and in which the guide walls locate to sealingly engage with the inner surface of the retentate side of the housing. wherein the crossflow filter device comprises: a filter membrane ; a flow channel for the pressurised feed liquid which extends in a path over a retentate surface of the membrane such that the direction of flow in the channel is tangential to the retentate surface, and a filtrate derived from the feed liquid passes through the membrane leaving retentate liquid in the flow channel; and a collection chamber for the filtrate formed on an opposite, filtrate surface of the membrane; wherein the crossflow filter device further comprises a sealed housing having a retentate side and a filtrate side which enclose therebetween the flow channel , the filter membrane and the collection chamber; and wherein the crossflow filter device further comprises flow channel guide walls provided at an inner surface of the retentate side of the housing, the guide walls being configured such that the path for the flow channel shuttles back and forth over the retentate surface of the membrane , characterised in that : the flow channel guide walls , which are formed of resiliently deformable elastomer, deform and sealingly engage with the filter membrane to form a fluid tight seal with the filter membrane and thereby define the path of the flow channel over the retentate surface of the membrane, and eliminate short-circuiting of the flow channel by the feed liquid; and the inner surface of the retentate side of the housing includes a recess corresponding to the line of the guide walls and in which the guide walls locate to sealingly engage with the inner surface of the retentate side of the housing. the flow channel guide walls , which are formed of resiliently deformable elastomer, deform and sealingly engage with the filter membrane to form a fluid tight seal with the filter membrane and thereby define the path of the flow channel over the retentate surface of the membrane, and eliminate short-circuiting of the flow channel by the feed liquid; and the inner surface of the retentate side of the housing includes a recess corresponding to the line of the guide walls and in which the guide walls locate to sealingly engage with the inner surface of the retentate side of the housing. <EOS>