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This work was designed to study the subsurface structures in the Siwa Oasis area of the Western Desert in Egypt and to determine their effects on surface geologic structures. A detailed land magnetic survey was performed in traverses covering about 400 km(2) of the study area. The measured total magnetic field was corrected and reduced to the north magnetic pole. The reduction-to-pole aeromagnetic and Bouguer anomaly maps were used to obtain regional extensions of these subsurface structures, study the continuation of these structures in sedimentary rocks, and delineate the depths to the basement rocks. Data analysis was performed using trend analysis, Euler deconvolution, Werner deconvolution, the Hilbert transform of the analytical signal, and 3-dimensional magnetic modeling methods. The results indicate that the area is affected by tectonic forces in the E-W, N45A degrees-65A degrees E, and N35A degrees-45A degrees W trends, which are correlated with the directions of surface geologic lineaments. The depth to the basement rocks increases from 3.2 km in the southern part to about 3.6 km in the northern part of the area. The results are in good agreement with depths obtained from drill hole data in the area.