The three dimensional plasma boundary displacements induced by either longlasting core MHD instabilities or by applied non-axisymmetric magnetic perturbations have been measured in ASDEX Upgrade, DIII-D, JET, MAST and NSTX. For the core instabilities, the displacements are usually small, although in extreme cases in MAST when the rotation braking is strong and global a significant displacement can be observed. The most concerning instability for ITER is the saturated internal kink, or helical core, which can be found in plasmas with a wide region of low magnetic shear such as the hybrid scenario, which can lead to non-negligible boundary displacements. Nonetheless, the boundary displacement resultant from core MHD instabilities in ITER is predicted to be less than ±1% of the minor radius. The displacements arising from applied resonant magnetic perturbations (RMPs) are measured up to ±5% of the minor radius in present day machines. Good agreement has been found between these experimental measurements and a range of models ­ from vacuum field line tracing, to ideal three dimensional MHD equilibrium reconstruction, to nonlinear plasma amplification. The measured displacement across a range of machines is found to correlate linearly with the applied resonant field predicted by vacuum modelling. The RMP-induced displacements foreseen in ITER are expected to lie within the range of those predicted by the different models, meaning less than ±1.75% (±3.5cm) of the minor radius in the H-mode baseline and less than ±2.5% (±5cm) in a 9MA plasma. Whilst a displacement of 7cm peak-to-peak in the baseline scenario is marginally acceptable from both a plasma control and heat loading perspective, it is important that ITER adopts a plasma control system which can account for a three dimensional boundary corrugation to avoid an n = 0 correction which would otherwise exacerbate the displacement caused by the applied fields.