New liquefaction susceptibility criteria for saturated silts and clays are presented that are based on the mechanics of their stress-strain behavior and which provide improved guidance for selecting engineering procedures for estimating potential strains and strength loss during seismic loading. Monotonic and cyclic undrained loading test data for silts and clays show that they transition, over a fairly narrow range of plasticity indices อPIอ, from soils that behave more fundamentally like sands อsand-like behaviorอ to soils that behave more fundamentally like clays อclay-like behaviorอ, with the distinction having a direct correspondence to the type of engineering procedures that are best suited to evaluating their seismic behavior. It is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine-grained soils exhibiting sand-like behavior, whereas the term cyclic softening failure be used to describe similar phenomena in fine-grained soils exhibiting clay-like behavior. For practical purposes, clay-like behavior can be expected for fine-grained soils that have PIเด 7, although a slightly lower transition point for soils with a CL-ML classification อperhaps PIเด 5 or 6อ would be equally consistent with the available data. Issues related to the practical application of these criteria are discussed.