This paper conforms to the usual concept of an embellished literature survey and as such is useful as a review of technology. In an area of such broad scope no authors can be expected to offer a meaningful assessment of differing effects from the so numerous variables. There is value in the paper as an up-dating of events since the notable publications "Advances in . Hydrocracking" (Advan. Petrol. Chem. Refining, 8, 169 (1964)) and "Hydrocracking To-day and Tomorrow" (Petro/Chem Eng., 37, 32-35 (Oct 1965)).