The recent discovery of gravitational waves has opened a new window in astronomy. Accreting systems are very common in astrophysics and are multifreqeuncy sources, since their emission spans the whole electromagnetic spectrum. Accreting systems in astronomy are also multi-messenger sources, since they can emit gravitational waves via a variety of physical mechanisms and because of their intrinsical binary nature. This paper presents a review of the gravitational wave emission of accreting systems at all scales, from cataclysmic variables to active galactic nuclei, showing that the emission occurs over the whole gravitational spectrum.