In Japan, in late 1989, the Environmental Agency's Central Council for Environmental Pollution Control instituted an exhaust gas control program for diesel vehicles, setting forth the permissible emission levels. The program not only imposes a substantial curb on NO x emissions but also calls for the reduction of black soot exhaust gas, which is peculiar to diesel vehicles, and restricts it in terms of the particulate matter (PM) emission level.Oil companies, on the other hand, have been required to reduce diesel oil's sulfur content, which corrodes the equipment and piping tubes and causes deterioration of the catalyst, from 0.4% (or 0.5% under the Japanese Industrial Standards) to 0.2% by 1993 and further to 0.05% by 1997.The overall performance of the deep hydrodesulfurization (HDS) process for diesel oil is advancing with both improvements in catalysis and developments in process design. The item most focused on in the process development is how to control the color of product diesel oil. Japanese consumers of diesel oil tend to object to the current color on grounds that it reminds them of erstwhile inferior-quality products. This market inclination has become an essential point of quality control. Some problems are also discussed concerning a scale up of reactor, reactor efficiency, reaction kinetics, catalyst life and hydrogen consumption.