We discuss atomistic aspects of the silicide contact reaction inferred primarily from STM observations of the prototype system Co/Si(111). For room temperature deposition and low coverage (0.01M1) we find that metal atoms exist as near-surface interstitials within the 7×7 reconstruction. Bond breaking associated with silicide formation occurs only at higher coverages. Deposition at 320C results in flat-topped triangular islands of epitaxial CoSi2 with a metastable 7-fold (111) interface, stabilized by the lateral silicon-silicide interface along the island edges. Some islands are covered with a 2×2 array of silicon adatoms. Very high temperature annealing (1200C) results in an 'impurity stabilized 1×1' surface which is in fact a lattice gas of ring-clusters that appear like tiny donuts or bagels in empty states STM images. These structures phase-separate from the clean 7×7 structure upon cooling below 850C.