Abstract: A sliding sash window assembly which comprises a fixed rectangular outer frame securable in a window aperture and having upper and lower window units (13, 14) mounted in the fixed outer frame and at least one of which is relatively movable between an open position and a closed position of the window assembly, and respective rectangular frames for the units arranged to permit lower horizontal frame member of the upper unit (13) to be located alongside an upper horizontal frame member of the lower unit (14) when the window assembly is closed, so that a locking element can lock the frame members to each other. The rectangular outer frame has side frames in which counterweight arrangements are housed, and forming a stack of weights which can be pulled upwardly within the housing by an elongate tensile element (20) which is secured at one end at or near the lower end of the stack, and at its other end is engageable with a lower end region of the movable window unit.