About 80000 X-ray sources with a detection likelihood 
were found during the ROSAT All-Sky Survey
(Voges et al. 1996b, 1997a), from which the 18811 brightest sources were
compiled in the ROSAT Bright Source Catalogue (RASS-BSC) (Voges et al. 1996a,b).
Cross-correlation with Galactic and extragalactic catalogues compiled in the
SIMBAD and NED database showed that more than 
of the objects are objects previously unknown. The identification of
these objects, and more to come with the release of additional catalogues,
is a challenge, as the X-ray data alone provide only limited insight
into their nature. Digitized optical sky surveys play a major role in this
effort to provide candidates for optical counterparts to the X-ray sources.
Despite the relatively small error radii (
) of the
RASS positions, often more than one candidate is
present and follow-up optical spectroscopy is needed to determine the
nature of the candidates.
The need of ample telescope time for follow-up spectroscopy can be
alleviated considerably by using the information provided by
(digitized) objective prism plates. The low-dispersion spectra allow to
draw conclusions on the nature of the optical candidates and
provide in many cases
unambiguous identifications of the X-ray sources. Therefore,
digitized plates of the objective prism survey for bright QSOs and direct
plates at the Hamburger Sternwarte (Hagen et al. 1995) were used
to provide identifications for X-ray sources
from the RASS. The identification process
covers currently 
deg2 of the high galactic latitude
northern sky
(337 fields from the Hamburg Quasar Survey (HQS,
Hagen et al. (1995),
each with an area of 
),
and with the present paper we release a first catalogue
containing optical counterparts for 3847 X-ray sources from the RASS-BSC.
Use of the identifications has been made already to study the RASS content
of AGN. From follow-up spectroscopy of 550 counterparts classified as AGN
candidates we found
a confirmation rate 
95% (Bade et al. 1992a). Subsequently,
samples of new ROSAT detected emission-line AGN (Bade et al. 1995;
Cordis et al., in preparation),
and BL Lac objects (Bade et al. 1994;
Nass et al. 1996)
were analyzed.
In the present paper we describe the identification technique (Sect. 2 (click here)), we discuss the classification criteria (Sect. 3 (click here)) and present the Hamburg/RASS Catalogue of optical identifications (HRC) (Sect. 4 (click here)). In the remaining sections the reliability of the identifications and the completeness of the catalogue are discussed, and statistics on its content are presented.