Publisher Summary This chapter discusses those properties of heavy feeds that are important for hydroprocessing, dewaxing, and residue fluid catalytic cracking (RFCC). From the chemical structure point of view, heavy feeds, such as vacuum gas oil (VGO) and heavy gas oil (HGO), are characterized by the three main hydrocarbon groups—that is, paraffinic, naphthenic, and aromatic. In addition, HGO contains alkenes because of the more severe conditions applied during coking than vacuum distillation. From the colloidal structure point of view, the heavy feeds, such as VGO, predominantly consist of the oil phase with resins accounting for a smaller portion. At the same time, resins may account for most of the DAO although this may depend on the efficiency of the solvent used for deasphalting. Asphaltenes are the main component of the asphalt by-product separated from heavy feeds although some resins may be still present. Again, the content of resins in the asphaltenes phase depends on the type of solvent used for precipitation of the latter, as well as on the origin of heavy feed. The feeds varying widely in viscosity may have similar specific gravity. This is not surprising, because different factors are involved in determining the values of viscosity and specific gravity. Therefore, in the case of heavy feeds, such as VRs and topped heavy crudes, viscosity is the parameter that has to be carefully monitored.