All types of interaction of a magnetized plasma flow with an obstacle (magnetized or not) are considered and those susceptible to produce a radio signature are identified. The role of the sub-Alfvénic or super-Alfvénic character of the flow is discussed. Known examples in the solar system are given, as well as extrapolations to star-planet plasma interactions. The dissipated power and the fraction that goes into radio waves are evaluated in the frame of the radio-magnetic scaling law, the theoretical bases and validity of which are discussed in the light of recent works. Then it is shown how radio signatures can be interpreted in the frame of the cyclotron-maser theory (developed for explaining the generation of solar system planetary auroral and satellite-induced radio emissions) for deducing many physical parameters of the system studied, including the planetary or stellar magnetic field. Recent detections of such radio signatures with new generation low-frequency radiotelescopes and future prospects are then outlined.