This paper considers the design of a life support system for transit to Mars and return to Earth. Because of the extremely high cost of launching mass to Mars, the Mars transit life support system must minimize the amount of oxygen, water, and food transported. The three basic ways to provide life support are to directly supply all oxygen and water, or to recycle them using physicochemical equipment, or to produce them incidentally while growing food using crop plants. Comparing the costs of these three approaches shows that physicochemical recycling of oxygen and water is least costly for a Mars transit mission. The long mission duration also requires that the Mars transit life support system have high reliability and maintainability. Mars transit life support cannot make use of planetary resources or gravity. It should be tested in space on the International Space Station (ISS). The oxygen and water recycling systems developed for the ISS are adequate for Mars transit but all have not yet been used on ISS and none were designed for the much higher mass launch costs and reliability requirements of Mars transit. Significant time and development effort, including long duration testing on ISS will be needed to provide the high quality oxygen and water recycling system that is needed for Mars transit.