Magnetic structures in thin-film recording media were imaged with sub-micron resolution while undergoing de erasure. Distinct changes in the patterns occurred at applied fields as low as 150 Oe below the media coercivity, and characteristic spatial variations were observed with applied fields approaching and extending beyond the coercivity. The results show that the onset of erasure is initiated by the rotation of magnetic moments at the track edges. As the field is increased, the regions magnetized parallel to the applied field expand in the direction of the field while those oriented antiparallel to it contact. Above the coercivity the unfavorably magnetized areas break up into remnant clusters whose sizes decrease as the entire region coalesces into a single magnetization structure. This paper correlates these microscopic results with macroscopic measurements of the medium and suggests possible mechanisms that lead to data erasure through media saturation.