A valve comprises a valve seat and a rotary valve element. The valve seat comprises a valve opening having a sealing surface. The valve element is mounted with respect to said valve seat for rotation about an axis of rotation between a closed position in which the valve element substantially closes the valve opening and an open position in which the valve element permits flow through the valve opening. The valve opening is elliptical in shape, having spaced apart upper and lower edges the upper and lower edges being generated as respective ellipses formed by parallel sections through a conical surface. The valve seat sealing surface is defined between said upper and lower edges.