Over the past 20 years, NASA has built and sent to Mars a series of orbiters, landers, and rovers designed to explore the red planet’s earliest history and seek signs of life. These missions discovered that in contrast to its current harsh environment, the ancient surface of Mars was wet, with a warmer climate, and thought to have been habitable by simple microorganisms. Nutrients, sources of energy, and all the key ingredients to sustain life appear to have been present. The remaining question for future missions is now to determine if life ever originated on Mars. The next rover mission, due to launch in 2020, will collect rock samples for return to Earth, where they can be examined with our most technologically advanced scientific instruments, giving us our best chance to date to search for fossils of ancient life.