ESA's Scout mission line is a component of FutureEO for small satellites that demonstrate new Earth-observation techniques and add scientific measurements alongside larger Earth Explorer missions. It uses rapid, lower-cost development to test concepts and involve industry and academic teams. The line includes HydroGNSS for hydrological climate variables, NanoMagSat for Earth's magnetic field and ionosphere, and Tango for greenhouse-gas observations. Individual Scouts can comprise several satellites. This programme brings together their shared science and technology-development approach.