  We describe the main features of the BMW survey of serendipitous X-ray
clusters, based on the still unexploited ROSAT-HRI archival observations. The
sky coverage, surface density and first deep optical CCD images of the
candidates indicate that this sample can represent an excellent complement to
the existing PSPC deep cluster surveys and will provide us with a fully
independent probe of the evolution of the cluster abundance, in addition to
significantly increasing the number of clusters known at z>0.6.
