The invention relates to a fuel processor that produces hydrogen from a fuel. The fuel processor comprises a reformer and a heater. The reformer includes a catalyst that facilitates the production of hydrogen from the fuel; the heater provides heat to the reformer. Multipass reformer and heater chambers are described that reduce fuel processor size. Single layer fuel processors include reformer and heater chambers in a compact form factor that is well suited for portable applications. Some fuel processors described herein place an electrically resistive material in contact with a thermally conductive material to heat fuel entering the fuel processor. This is particularly useful during start-up of the fuel processor. Fuel processors described may also include features that facilitate assembly.