Designs for future exploration of Mars that include the use of resources obtained on the martian surface can greatly expand the range of exploration compared with those that reply only on material carried from Earth. In addition, the use of in situ resources provides a step in the direction of a self-sufficient settlement. The key resources are: O2, H2O and buffer gas (either N2 or N2/Ar). Promising laboratory scale prototypes are already under development and the production of oxygen may be practical even on near-term robotic missions. Preliminary calculations suggest that water will be difficult to produce on Mars. This detriment is partially offset by the ease of storage of water. Buffer gas is a requirement for a breathable gas mixture and either N2 or N2 /Ar gases can be produced from the martian atmosphere. Two important criteria for judging systems for the production of resource on Mars are: 1) the mass gain factor, which is the total mass of resource produced by the system divided by the mass of the system, and 2) the specific energy required to produce a unit mass of the resource. To be practical the mass gain must be 100 or more and the specific energy must be less than about 10 kW-hr/kg.