The first missions to Mars may be followed by more extensive visits. Mission duration must comply with launch windows and time spent on the Martian surface would typically amount to 500 days. A safe and comfortable habitat will be required, along with a reliable supply of all necessary life-support materials (e.g., breathable air, potable water, food). Consumables transported from Earth will be recycled extensively. Water can also be mined on site, both for use as such and to generate oxygen. The latter could be used for breathing, but also for combustion fuel, both as a local energy source and for the return journey. Early crew habitats may gradually develop into long-lasting scientific outposts. Its most important tasks would be the scientific exploration of the geological and (putative) biochemical history of the terrain and to enable a more sustained presence by prospecting for in-situ resources. Such stays would pose unprecedented challenges to the crew’s physical and psychological endurance. It will also raise major legal and ethical questions, which require serious consideration before any such mission takes place.