The measurement of frequencies, amplitudes and lifetimes of solar oscillations has proven to be a very powerful tool to sound the properties of the solar internal structure. Combined with observations of emerging solar magnetic field parameters of the solar internal structure and dynamo can be inferred. The same techniques may be used to sound stellar interiors: asteroseismology and observation of magnetic activity (emerging magnetic fields). The observation of photometric fluctuations down to micromagnitudes on stars along or near the main sequence when complemented with measurements of activity levels will provide a wealth of information on the internal structure and rotation rates, and its evolutions with age. The PRISMA mission is an ESA Assessment Study currently underway. We present here the scientific objectives of the mission, the measurements to be made, the techniques to be used, and the main characteristics of a model payload.