I review our observational and theoretical knowledge of AM CVn stars,
focusing on recent developments. These include newly discovered systems, the
possibility that two recently discovered extremely short period objects are AM
CVn stars and an update on X-ray, UV an optical studies. Theoretical advances
include the study of the details of both the donor and accretor, and the
physics of the helium accretion discs. I review our (limited) knowledge of the
formation of AM CVn stars and the apparent success of the now more than 25 year
old suggestion that in these objects the mass transfer is driven by
gravitational wave radiation losses. The exciting prospect of directly
detecting these gravitational waves and the possibilities this brings conclude
this contribution.Comment: Error in table 1 corrected: orbital period GP Com is 2794 sec. Review
to appear in "The astrophysics of cataclysmic variables and related objects",
ASP Conf. Ser.,eds. J.M. Hameury and J.P. Lasot
