The predominance of extended zones of linear anomalies as well as of curved, ring-shaped, and other complicated anomalies, is characteristic of the magnetic field over ancient shields and platforms. The zones of linear anomalies reflect large tectonic features of the ancient geosynclines—deep faults and the associated structural facies zones. Curved anomaly zones characterize regions of deep-seated folding which are peculiar to ancient shields and platforms.Three large groups of deep structures may be distinguished in terms of the magnetic effects they produce. The first group includes domes (arches) composed essentially of nonmagnetic granitoids surrounded by stable magnetically active metamorphic zones. Structures of this group are most extensively developed in the inner zones of ancient geosynclines.A characteristic feature of the second group is magnetic anomalies over migmatite domes, whose minima extend over bordering synclines composed of nonmagnetic rocks. The trend of individual magnetic horizons is concordant with the direction of banding in the migmatites.The third group includes deep structures whose synclinal and anticlinal forms are composed of rocks showing an increased magnetization. Mapping of such structures in the magnetic field presents considerable difficulties.The autocorrelation functions of anomalous magnetic fields over ancient shields indicate the intricate nature of the fields. A zero-correlation radius of a high-frequency component ranges from 5 to 10 km; that of a long-wave component amounts to about 40 km.Two layers corresponding to the sources of anomalies are identified in the crustal cross section. The depths to lower boundaries of the bodies responsible for the local magnetic anomalies have shown that most of the sources are confined to a surface layer which is not more than 10 km thick. The source of regional magnetic anomalies is attributable to magnetic inhomogeneities of the earth's crust, lying at depths from 10 to 30 km.