The origin of the synchrotron radiation from extragalactic jets is shown to be the interaction between the jet and the insterstellar medium.This result is derived from a synthesis of available observationswhich strongly indicates that the one-sidedness is an intrinsic asymmetry. A plausible theoretical scheme is established and leads to the conclusion that the visible jet is interacting {\it less} with the interstellar medium than the invisible one (counterjet), contrary to a first intuition. The density asymmetry of the insterstellar medium required by the theory is in agreement with the observations. This model does not only account for one-sidedness of extragalactic radiosources but more broadly for double sources with no or two visible jets, for the small number of optical jets, and for some other properties of radiosources.