AM CVn systems are a subclass of cataclysmic variables where two DB WDs exchange H-depleted material under short period orbits -in the range approx. 5 - 65 minutes. The prototype, AM CVn (=HZ9) itself, was first discovered in 1957 as a "peculiar" DB WD by Greenstein & Matthews and in 1967 Smak found its characteristic 17-18 min photometric variability. The class has grown from 4 known objects in the nighties till 70 nowadays. Also, this class includes objects displaying superhump periodicities, magnetic accreators and some eclipsing systems, with a broad general range of photometric variability. So far, four AM CVn subclasses have been determined: 1) very short period systems, with Porb < 10 min, the so-called 'direct impact' accreators, have very high accretion rates, with no accretion disk and displaying X-ray emission; 2) bright objects with 10 min < Porb < 20 min, that have permanent high mass accretion regimes, dominated by the accretion disk; 3) those with 20 min < Porb < 40 min, that display variable brightness states corresponding to low/high mass accretion regimes; and 4) faint objects with Porb > 40 min, that correspond to permanent low mass accretion regimes, mainly dominated by the primary WD accretion.