Colloidal gels can undergo a solid to liquid transition, and the yielding and subsequent microstructural reorganizations are responsible for the complex rheological properties of these materials. In the present work, video microscopy is used to study the details of the microstructure during the yielding transition, using planar monolayers of model aggregated suspensions subjected to homogeneous interfacial shear flows. The microstructural heterogeneity of the quiescent structure on both local and larger length scales is characterized in detail. The pertinent length scales can be directly linked to the aggregation pathway. The large scale heterogeneity in the microstructure, rather than the local scale properties, determines the length scales observed during the initial yielding of the gel structure. The break-up and subsequent reaggregation leads to a local compaction and increasingly more heterogeneous structure. As the surface coverage increases, the heterogeneity and deformations become more and more localized. The experiments shed light on the pertinent length scales in colloidal gels and how they evolve upon initial yielding, which should be relevant for understanding the thixotropic response and guiding modeling efforts of the macroscopic rheological behavior of flocculated suspensions. (C) 2009 The Society of Rheology. [DOI: 10.1122/1.3237154]