The materials selection for FCCU Regenerator and Reactor (or Disengager) cyclones has always been a complex decision, and recent trends have led to even more factors to be considered. Often, the selection is a “least evils” decision. There are multiple materials degradation mechanisms, which vary depending on the specific unit conditions, and no one material completely resists them all. Operating temperatures have been increasing in recent years, which has added to the materials problems. In addition, other trends, including the use of partial combustion, close-coupling of cyclones, etc., have increased the complexity of the FCC process. This paper discusses the applicable materials degradation mechanisms, highlighting the operating conditions under which they become critical and the susceptible materials. The recommended materials and some comments on life expectancy for common plant conditions are then given.