As fuel injection pressures keep rising, questions focus on additional benefits to be gained from the considerable efforts to achieve and handle the fuel pressure increments. The aim of fuel injection processes is to support the mixing of fuel molecules with oxygen. The steps towards this goal include fuel atomization, evaporation, heat transfer from air into the liquid or vaporized fuel together with transport of fuel for best air utilization. Engineering degrees of freedom include the parameters of the fuel injection system and handling of in-cylinder gas conditions. The paper describes basic high pressure flow processes, spray propagation, evaporation and combustion and the mixing of flame clouds with in-cylinder air for oxidation of high temperature soot particles. Experimental evidence of such processes is derived from laboratory flow tests as well as from optically accessed engines operated under conditions relevant for todays passenger car and heavy duty engines.