Results from an array oftheoretical and computational tools developed to treat theinstabilities of most interest for high performance tokamak dischargesare described. The theory and experimental diagnostic capabilitieshave now been developed to the point where detailed predictions can beproductively tested so that competing effects can be isolated andeither eliminated or confirmed. The linear MHDstability predictions using high quality discharge equilibriumreconstructions are tested against the observations for theprincipal limiting phenomena in DIII-D: L mode negative central shear(NCS) disruptions, H mode NCS edge instabilities, and tearing andresistive wall modes (RWMs) in long pulse discharges. In the case ofpredominantly ideal plasma MHD instabilities, agreement between thecode predictions and experimentally observed stability limits andthresholds can now be obtained to within several per cent, and thepredicted fluctuations and growth rates to within the estimatedexperimental errors. Edge instabilities can be explained by a newmodel for edge localized modes as predominantly ideal instabilitieswith low to intermediate toroidal mode number. Accurate idealcalculations are critical to demonstrating RWM stabilization by plasmarotation, and the ideal eigenfunctions provide a good representationof the RWM structure when the plasma rotation slows. Idealeigenfunctions can then be used to predict stabilization usingactive feedback. For non-ideal modes, the agreement in some cases ispromising. Δ' calculations, for example, indicate that some dischargesare linearly unstable to classical tearing modes, consistent with theobserved growth of islands in those discharges. Nevertheless, thereis still a great deal of improvement required before the non-idealpredictive capability can routinely approach levels similar to thosefor the ideal comparisons.