Fatigue cracks often can be (more or less) tightly closed. Under such “closure” conditions the crack may be transparent to ultrasound and actually form electrical contact between fracture surfaces, diminishing the eddy current response with respect to the fully open crack. In addition, external environments, which affect the crack propagation rate, also change the state of closure. The present paper reports on a recently developed model which seems to describe this closure phenomenon and aids in the interpretation of NDT data such that the crack size can be determined even for tightly closed cracks.