This chapter describes the SMILE (Solar wind Magnetosphere Ionosphere Link Explorer) mission, currently under development as a joint project of the European Space Agency and the Chinese Academy of Sciences. SMILE aims to study the solar wind coupling with the terrestrial magnetosphere in a very novel and global way, by imaging the Earth's magnetosheath and cusps in X-rays emitted when high charge-state solar wind ions exchange charges with exospheric neutrals. SMILE combines this with simultaneous UV imaging of the northern aurora and in-situ plasma and magnetic field measurements of the magnetosheath and solar wind while flying in a highly elliptical northern polar orbit. In this chapter, after a brief introduction to the science that SMILE is targeted to investigate, the payload and the spacecraft, and progress in their development, are described. The mission operations in space, the ground segment and the scientific preparations that will ensure the optimal exploitation of SMILE measurements are also presented. The international space plasma and planetary communities are looking forward to the step change that SMILE will provide by making visible our invisible terrestrial magnetosphere.

This Chapter will appear in the Section "X-ray Missions" (Section Editors: Arvind Parmar, Andrea Santangelo, Shuang-Nan Zhang) of the "Handbook of X-ray and Gamma-ray Astrophysics" by Springer (Editors in chief: C. Bambi and A. Santangelo)