Fuel processing is the umbrella term for a variety of chemical processes aiming at the provision of a hydrogen-rich gas mixture to be fed into the anode of a fuel cell. Among them, the most important ones are desulfurization, reforming, water-gas-shift reaction, preferential oxidation, and catalytic combustion. At Juelich, all these processes are under investigation. This contribution concentrates on autothermal reforming (ATR) of different synthetic and fossil fuels with Juelich’s reactor ATR 13. ATR 13 was developed within the European project Fuel Cell Based Power Generation (FCGEN). Here, special emphasis is on using diesel fuel SD10. SD10 is a conventional low-sulfur diesel, currently used for truck engine testing. ATR 13’s power class of 3–5 kWe is particularly interesting for fuel cell systems with polymer electrolyte fuel cells (PEFCs) working as auxiliary power units (APUs) for onboard power supply in trucks. During autothermal reforming, liquid fuels such as diesel are converted together with air and steam into a hydrogen-rich gas by means of catalysis. The design and construction of ATR 13 was particularly optimized with respect to its power density, since ATR 13 will be assembled in an allocated and limited space at the bottom of a conventional truck. As a result, a high value of 3.6 kW/l was reached. Intensive experimental evaluation of ATR 13’s operational behavior was done and will be reported in this chapter.