The objective of this paper is to characterize the quasi-static and fatigue response of concrete subjected to biaxial stresses in the t-C-T region, where the principal tensile stress is larger in magnitude than the principal compressive stress. An experimental investigation of material behavior is conducted. The failure of concrete in the stated biaxial region is shown to be a local phenomenon under both quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the principles of fracture mechanics. It is observed that crack growth in constant amplitude fatigue loading is a two-phase process: a deceleration phase followed by an acceleration stage. The quasi-static load envelope is shown to predict the crack length at fatigue failure. A fracture-based fatigue failure criterion is proposed, wherein the fatigue failure can be predicted using the critical mode I stress intensity factor obtained from the quasi-static response. A material model for the damage evolution during fatigue loading of concrete in terms of crack propagation is proposed. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the biaxial fatigue response. © 2002 American Concrete Institute. All rights reserved.