The introduction of inertia terms into a theoretical model of thin ring expansion shows that classical plastic instability concepts, defined in terms of the local strain in the necking region, no longer apply. By introducing the role of void growth in triggering local necking, through a recently proposed constitutive equation for porous plastic materials, it is possible to overcome this difficulty. A criterion based on a critical void volume fraction provides the alternative means to predict failure. This theory was applied to a thin ring configuration, and the results were found to compare favorably with experimental values obtained from statically and dynamically loaded rings. The experimental techniques developed to handle the thin ring configuration are also described.