It has been estimated that the Earth has nearly 1.688 trillion barrels of crude oil, which will last 53.3 years at current extraction rates. The organization of petroleum exporting countries (OPEC) group forecasted that the oil prices will not jump to triple-digit territory within a decade, but it can quickly increase as the political issue for reducing oil production appears. With the potential of serious shortage of conventional hydrocarbon resources, the heavy oil, one of unconventional hydrocarbon resources including oil sand and natural bitumen has attracted worldwide interest. The heavy oil contains heavy hydrocarbon compounds, commonly called as resins and asphaltenes, with long carbon chains more than sixty carbon atoms. The high content of heavier fraction corresponds with the high molecular weight, viscosity, and boiling point. Physicochemical properties of residues from vacuum distillation of conventional oil, referred to as vacuum residues (VR) were similar to those of heavy oil. For the development of heavy oil reserves, reducing the heavy oil viscosity is the most important. In this article, commercially employed aquathermolysis processes and their application to VR upgrading are discussed. VR contains transition metals such as Ni and V, but these metals should be eliminated in advance for further refining. Recent studies on demetallization technologies for VR are also reviewed.