Euclid is an astronomical and astrophysical ESA space mission. The launch of the mission is scheduled for 2021. The goal of the Euclid mission is to better understand why the expansion of the universe is being accelerated, and to get to the bottom of the cause of expansion that is suspected in the dark energy. Dark energy represents approximately 75% of the energy content of today's universe. Together with dark matter, it dominates the matter-energy content of the universe. Dark energy and dark matter are both mysterious and of unknown nature, but they control the past, present and future evolution of the universe. Euclid will explore how the universe has evolved over the last 10 billion years to answer fundamental physics and cosmology questions about the properties of dark energy, dark matter and gravity.