STARS is an asteroseismology mission, which, through the acquisition of very accurate light curves, will detect oscillations in a wide variety of stars. This will allow, for the first time, the internal structure of stars of different age, composition and spectral type, to be studied directly, bringing the spectacular successes of helioseismology within reach for a large number of other stars spreading over most of the Hertzsprung-Russell diagram, including solar-type stars in open clusters. The results of the phase A study are presented in this report.