Fundamental understanding of electric field mediated assembly of interfacial colloids provides the ability to design, control, and optimize a number of emerging particle based materials and devices. This talk will present measurements and models of the assembly of colloids in external AC electric fields in interfacial electrode geometries. Results will be presented for video and confocal microscopy measurements of micron sized silica colloids in aqueous media. In this work, dispersions were manipulated by ~MHz AC electric fields using an interdigitated electrode array with gap separations on the order of tens of microns. High-frequency fields ensure that dielectrophoresis dominates electrophoretic and electroosmotic transport mechanisms. Radial and two-dimensional distribution functions are used to characterize microstructures and to quantitatively connect them to particle-particle, particle-surface, and particle-field interactions. Experimental results are also directly compared with Brownian dynamic and Monte Carlo simulations. For applied fields above a certain threshold, dispersions form polycrystalline solids that exhibit a high degree of local six-fold ordering. Progressively weaker fields exhibit structures that resemble dipolar chain-fluids. At still weaker fields, thermal fluctuations dominate field driven assembly and associated fluids transition to weakly perturbed fluids. Based on our findings, we are able to demonstrate the use of oscillatory electric fields for mechanical annealing of defects in colloidal crystals.