This thesis addresses the seismic evaluation of existing buildings. In particular, it focuses on the seismic behavior of lightly reinforced shear walls that are not designed to withstand earthquake actions. A shear strength envelope for the assessment of deformation capacity of these non-ductile walls is presented. The approach is the result of experimental investigations and analytical modeling. Existing models for plastic hinges in beams are enhanced in order to determine drift capacity of lightly reinforced concrete shear walls. The static-cyclic behavior of non-ductile, reinforced concrete shear walls is investigated by testing four small-scale specimens of shear span ratio equal to 0.8. The design of the specimens includes reinforcement ratios, and axial force levels in existing shear wall buildings. Although the specimens were expected to fail in brittle shear, low to moderate ductile response is obtained. The deformation capacity, not the shear strength, is found to be restricted by shear failure. It is observed that inherent shear strength of concrete and the concrete compression zone are the principal contributors to the shear capacity of lightly reinforced shear walls. It is also observed that low reinforcement ratios and moderate levels of axial force can efficiently prevent brittle response in shear. The analytical model consists of a plastic hinge over the entire height of the low-rise shear wall. Proposals are made for the strain distribution inside the plastic hinge. Explicit relationships between drift and base shear are established and it is found that the model accurately predicts the envelope curve of static-cyclic loading. The shear strength envelope is formulated by using the analytical model. Criteria for the failure modes of diagonal tension, of concrete crushing, and of sliding enclose the shear strength envelope. In addition, inherent shear strength forms the lower bound of this envelope. The contributions of reinforcement and concrete to shear capacity are formulated in terms of initial strength and strength decay. Accurate prediction of both the ductility supply and the drift capacity obtained in static-cyclic tests is observed. Validation of the shear strength envelope on full-size walls prevalent in existing buildings shows potential for further application. The proposal contributes to more realistic evaluation of shear strength in selected situations where available methods are too conservative. Hence, it allows for both avoiding costly seismic strengthening in such situations and better allocation of resources where they are really needed.