This thesis describes the structural performance of plywood sheathed timber shearwalls exposed to simulated seismic loading and proposes a simple design method. Five full sized three-storey shearwalls were constructed using timber framing and plywood sheathing. The shearwalls were of high aspect ratio and one had large openings. A variety of framing arrangements and hold-down details were used. Each of the shearwalls was instrumented and subjected to reverse cyclic loading representative of the fundamental mode of seismic loading. Construction of the walls and their behaviour under load is described in detail, with an analysis of stiffness, strength, ductility and failure modes. Material properties of the shearwall components were investigated experimentally. A capacity design method is proposed to prevent brittle fracture of the timber and verified by reliability analysis. A proposed method of calculating deflected shapes gives good agreement with test results. The seismic response of multi-storey buildings is investigated. A structural design procedure and suitable detailing for multistorey timber shearwalls incorporating light frame timber construction and plywood sheathing is developed. The procedure is specifically intended for the design of timber shearwalls resisting NZS 4203:1992 seismic loading requirements and meeting NZS 3603:1993 design criteria, but can be adapted to meet other codes of practice. Capacity design methods are incorporated. An example design of a three storey shearwall is included.