Euclid is a space mission of the European Space Agency to characterize dark energy and dark matter and is poised for launch in 2022. Euclid is designed to provide measurements for two dark energy probes, Weak Lensing and Galaxy Clustering, enabling sufficient precision for a decisive test of the Lambda CDM model. The mission will carry out simultaneously high precision imaging, deep imaging photometry in the infrared Y, J, and H bands, and infrared slitless spectroscopy in a survey area of 15,000 deg2. I will present the science case and the flow of the requirements driving the design of the mission. An overview is given of the designs and as-built performances of the 1.2-meter telescope, instruments, and spacecraft that have been optimized to achieve the cosmological goals. The nominal survey will last for 6 years and special attention is given to the construction of the survey, consisting of wide and deep survey components. The potential legacy of Euclid is unprecedented, the Euclid Collaboration has set up an extensive data processing and archive infrastructure to optimize the production and to maximize the scientific exploitation of the Euclid data. An overview of topics is given to highlight the scientific prospects of the mission. Euclid will be one of the many big surveys of this decade, forcing us to think of new ways to maximize the scientific synergies.