Out of plane instability of reinforced concrete walls is a failure mechanism observed rather recently following the earthquakes in Chile (2010) and New Zealand (2011). Global out of plane instability is a phenomenon that may occur in thin walls, subjected to in plane cyclic loading, which develop large out of plane deformations over the storey height. Past experimental efforts to study this phenomenon focussed on thin members with two layers of longitudinal reinforcement. Hence, also the phenomenological models proposed in literature were mainly based on experimental evidence from double layered members. However, a constructive boom in Latin America, together with an elevated cost of materials, has led over the last fifteen years to the massive construction of reinforced concrete buildings in which the shear walls are very thin and have a single layer of reinforcement. Given the lack of experimental data on single layered members made it questionable whether the mechanism develops as in double layered members, and if the existing models are capable to reproduce the response. In view of the above, the present thesis has the two following goals: (i) Provide new experimental insight on the out of plane instability of members with a single layer of reinforcement; (ii) Develop and validate numerical and mechanical tools to simulate and assess the vulnerability of thin walls to out of plane buckling. The experimental findings obtained from two experimental programs, on thin walls and isolated wall boundary elements respectively, are presented. The importance of the maximum (or critical) tensile strain experienced by the member to trigger instability, which was already identified in the past, is supported by the experimental results. Nevertheless, discrepancies from the usual hypotheses assumed for members with two layers of reinforcement are pinpointed, in particular with respect to the buckling height involved in the deformation, and to the magnitude of the axial force at onset of out of plane instability. Moreover, the crucial role played by the crack pattern is discussed, and an innovative failure criteria based on a three hinge mechanism related to the compressive strains attained in the cracks is identified. Finally, also the adverse effect of an imposed out of plane displacement at the top is also addressed. For a more thorough understanding of the phenomenon, and with the intent of developing simple methods to calculate the critical tensile strain triggering out of plane failure, numerical models are developed and validated against experimental results. The simulations allowed identifying important differences between the isolated boundary element and the wall behaviour, in particular with respect to the vertical displacement profile along the height. On the basis of the numerical findings, an improved boundary element model, rather simple to use for assessment and design, is presented. Finally, building on the experimental and numerical findings, a new mechanical is proposed. The model is in principle developed for members with one layer of reinforcement, but it is shown how it can be extended to double layered members. The mechanical model sets apart from existing ones as it assumes different curvature and vertical displacement profiles along the entire storey height, it accounts for axial forces lower than yielding in compression, and it allows to consider various boundary conditions.