Satellites constellations and formations are becoming a growing segment of the space market due to numerous possibilities to provide new, effective commercial and scientific services and applications. Satellite constellations offer a base for communication, monitoring, and navigation services, while satellite formations tie into the move towards disaggregated satellite design. Flying satellites in coordinated formations have advantages over single monolithic spacecraft: higher redundancy, unprecedented high resolution, and the ability to view research targets from multiple angles or at multiple times. These qualities make formation flying particularly suitable for astronomy, communications, meteorology, and environmental issues, to name some applications. From previously flown missions like PROBA-3, GRACE (Gravity Recovery and Climate Experiment) or Cluster II to more recent mission concepts using small satellites, formations have demonstrated to be an enabling technology to achieve objectives that would not be possible with single monolithic spacecraft.