• Everyone wants to measure coronal magnetic fields: flares, coronal heating, CMEs, …. • Most of us come from Flatland: we are accustomed to 2D photospheric field measurements. •The corona is different: intrinsically 3D. Have to think differently. Can’t measure 3D fields with an optically thin diagnostic; here radio has an advantage because it is optically thick: get different layers of the corona at different frequencies. • Extrapolations of surface fields will be just as crucial because we don’t measure the height of the gyroresonance layers. • But extrapolations need radio measurements of coronal field strengths because the photospheric and chromospheric measurements are incomplete.