The Solar TErrestrial RElations Observatory (STEREO) is a two-spacecraft NASA-led mission to study the three-dimensional structure of the Sun and heliosphere. STEREO's primary scientific objectives are to understand the mechanisms by which material leaves the Sun in both large eruptions called coronal mass ejections (CMEs) and the steady flow known as the solar wind, and to determine how this material propagates through the solar system. One of the two spacecraft that comprise the mission orbits the Sun ahead of the Earth, while the other trails the Earth. The separation between the two spacecraft allows them to capture stereoscopic images of the Sun and make in situ measurements that reveal the three-dimensional structure of interplanetary CMEs and the solar wind.