An advanced model has been developed to calculate interactions between plasticity and roughness closure mechanisms resulting in the fatigue crack growth threshold. Near-threshold closure is complex because multiple closure mechanisms are likely, including roughness-, oxide-, and plasticity-induced crack closure. Results show closure first occurs either at the crack tip (tip contact), or at the fatigue-crack-surface asperity nearest the crack tip (asperity contact), in the absence of load history effects. Model calculations, verified by laboratory experiments, reveal a transition from tip contact to asperity contact produces fatigue crack growth threshold behavior; here, both model and experiment show that an abrupt increase in closure (asperity contact) results in the rapid decrease in fatigue crack growth rate at threshold. A comparison of an advanced crack-tip load displacement measurement technique, and standard global compliance methods show that global methods lack the sensitivity to accurately characterize crack closure at threshold.