Many high temperature alloys are based on Fe, Ni and Co with significant levels of chromium added for corrosion resistance. During service in carbon-rich environments, such alloys can degrade by two distinct corrosion processes. One is carburization, which generally occurs at temperatures in the range, 800 - 1100 degrees C, while the other is metal dusting which typically manifests itself in the range, 400 - 800 degrees C. In the present paper the sequential stages of alloy degradation when exposed to mixed carburizing-oxidizing environments having a carbon activity of similar to I are discussed. Four distinct stages of microstructure evolution are described. In the final stages, carbon diffusion into the alloy interior followed by the precipitation of stable, brittle carbide phases affect the mechanical integrity of the material. By contrast metal dusting is a process that occurs in carbon- supersaturated environments (carbon activity > 1) and results in the actual conversion of bulk metal to powder or dust. The mechanistic aspects of metal dusting are discussed with particular attention to the behavior of Fe.