Abstract: A recent upgrade of the DIII-D tokamak magnetic diagnostic is providing new insights to our understanding of the plasma response to external non-axisymmetric (3D) perturbations This talk will highlight two challenges for 3D control in tokamaks: 1) correction of long wavelength fields for error field (EF) control, and 2) application of short wavelength fields for edge-localized mode (ELM) control. Correlation of optimal control with 3D response measurements identifies the underlying tokamak linear instabilities mediating EF and ELM control - and reveals their dissimilarity. While linear physics governs which responses are important, at their extreme both EF and ELM control fields trigger similar non-linear bifurcations. This highlights the potential of driven reconnection (3D field penetration) to provide a common physical framework for both effects.