The Self-Nulling Eddy Current Probe has been the focus of much research during the past several years [1–7]. Developed under NASA’s Airframe Structural Integrity Program, past research has focused on applying the Self-Nulling Probe technology to the inspection of damage to thin aluminum airframe skins. As a result of this work prototype fatigue crack detectors, single and multi-layer thickness gauges, and a system for the detection of cracks under installed fasteners have been developed[l–2,5–7]. The probe has also been successful at detecting surface flaws in thick bulk materials, for which a commercial instrument has been produced and marketed by Kramer Branson, Inc.-This paper will explore the fatigue crack detection mechanism of the Self-Nulling Probe for shallow flaws in thick materials as compared to that of through cracks in thin skins. The resulting change in the performance of the Self-Nulling Probe will then be detailed, and proposed modifications to optimize the performance of the probe for the detection of shallow fatigue cracks enumerated.