This article is a selective review of new approaches and emerging technologies for ultra-clean (ultra-low sulfur) diesel fuels. The issues of diesel deep desulfurization are becoming more serious because the crude oils are getting higher in sulfur content, while the regulated sulfur limits are becoming lower and lower. Deep reduction of diesel sulfur (from 500 to 15 ppmw sulfur) is dictated largely by 4,6-dimethyldibenzothiophene, which represents the least reactive sulfur compounds that have substitutions on both 4- and 6-positions. The deep hydrodesulfurization (HDS) problem of diesel streams is exacerbated by the inhibiting effects of co-existing poly aromatics and nitrogen compounds in the feed as well as H2S in the product. New and more effective approaches and continuing catalysis and processing research are needed for producing affordable ultra-clean diesel fuels, because meeting the new government sulfur regulations in 2006-2010 is only a milestone. Desulfurization research should also take into consideration the fuel-cell fuel processing needs, which will have stringent requirements on desulfurization.