In addition to conventional hydrocracking, the refining industry has used a milder form of hydrocracking over the last several years [1-5]. Mild hydrocracking is a process used to produce relatively high yields of lighter products by using operating conditions similar to those used for vacuum gas oil (VGO) desulfurization. Generally, mild hydrocracking is a conversion of VGO desulfurization units being operated at more severe conditions, i.e., higher temperatures. The general features of conventional and mild hydrocracking are compared in Table 12.1 [5-7]. The major difference between the two processes is the hydrogen partial pressure at which they operate; mild hydrocrackers operate on a once-through basis whereas most conventional hydrocrackers recycle the unconverted material. While conventional hydrocracking is carried out at hydrogen partial pressures varying from 100-180 bars (1465-2625 psig), mild hydrocracking uses pressures of 30-70 bars ( 450-1030 psig). The lower hydrogen partial pressure in mild hydrocracking leads to lower conversion and to less hydrogenated products. It produces a large quantity of low-sulfur fuel oil and smaller quantities of middle distillates. Furthermore, the total conversion of feedstock that can be achieved with conventional hydrocracking cannot be achieved with mild hydrocracking (see also Section 11.3.2).