At regional scales, electrical resistivity illuminates Earth processes involving fluid evolution and transport, temperature contrasts, and fault characteristics and behavior. It also clarifies continental terrane assembly and event sequencing through electronic mineral markers. Magnetotellurics (MT) is sensitive over such scales, but faces high property contrasts, small signals, 3-D complexity, discontinuous fields, and ill-posed inversion. So-called wideband (~0.003 – 500 s wave period) MT recording constrains crustal structure, and high fidelity through its central dead band is routinely achieved now via distant remote referencing, continuous streaming, and outlier removal. To resolve across the upper mantle, long period data must be of high quality through 10,000 s. Electronics modifications now permit good quality MT data over polar ice-covered regions, and non plane wave outliers appear largely avoidable. Regularized 3D non-linear inversion using simulation equations that recognize a spatially discontinuous electric field has become common practice and lends essential credibility to interpretations. However, resistivity model non-uniqueness is seldom tested enough, and assuming isotropic resistivity can lead to artifacts. Fluids interpreted to cause low resistivity in ductile deep crust should be at lithostatic pressures and have compositions compatible with ambient temperature and metamorphic grade. Vertical current channeling enhances resolution of large-scale fault zones connecting deep and shallow structures. Stabilized terranes can exhibit strong, quasi-linear conductors marking belts of graphite or sulfides deposited in sediment-starved foredeeps or rift margin basins, with a particular concentration in the Proterozoic corresponding to atmospheric oxygenation events. An exciting recent avenue is estimating H2O content of nominally anhydrous minerals (NAMs) in the upper mantle, which strongly affects electrical conductivity but not seismic velocity. The large bandwidth of MT data affords a broad-scale, unified view of Earth processes from mantle level sources through crustal storage and evolution to near-surface deposition. Support has been from U.S. Dept of Energy contract DE-0006732 and National Science Foundation grant OPP-1443532, and numerous prior.