AM CVn stars

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.