A multiaxial fatigue damage criterion has been formulated based on observations that fatigue crack development is governed by stresses and strains acting on critical planes in materials. The damage parameter incorporates both Mode I and Mode II loading. Failure is defined as the development of an engineering size crack (approximately 1 mm in surface length). The proposed method can be applied to both proportional and nonproportional multiaxial loading as well as to uniaxial loading. Predictions based on the proposed method were compared to uniaxial and multiaxial in-phase and out-of-phase test data for three materials. The approach demonstrated improved life predictions relative to other methodologies.