The Tycho-2 Catalogue is an astrometric reference catalogue containingpositions and proper motions as well as two-colour photometric data forthe 2.5 million brightest stars in the sky. The Tycho-2 positions andmagnitudes are based on precisely the same observations as the originalTycho Catalogue (hereafter Tycho-1; see Cat. )) collectedby the star mapper of the ESA Hipparcos satellite, but Tycho-2 is muchbigger and slightly more precise, owing to a more advanced reductiontechnique. Components of double stars with separations down to 0.8arcsec are included. Proper motions precise to about 2.5 mas/yr aregiven as derived from a comparison with the Astrographic Catalogue and143 other ground-based astrometric catalogues, all reduced to theHipparcos celestial coordinate system. Tycho-2 supersedes in mostapplications Tycho-1, as well as the ACT (Cat. ) and TRC(Cat. ) catalogues based on Tycho-1. Supplement-1 listsstars from the Hipparcos and Tycho-1 Catalogues which are not inTycho-2. Supplement-2 lists 1146 Tycho-1 stars which are probably eitherfalse or heavily disturbed. For more information, please consult theTycho-2 home page: http://www.astro.ku.dk/~erik/Tycho-2 (4 data files).