HORuS is a high-resolution (R=25000) spectrograph for the 10m Gran Telescopio Canarias (GTC). It collects light at the Nasmyth focal plane, shared with OSIRIS, using a 3x3 IFU (2.1x2.1 arcsec) with microlenses, into optical fibers that form a pseudo-slit at the spectrograph entrance. The light is dispersed with a 79 gr/mm echelle grating and cross-dispersed with three prisms, providing almost continuous coverage between 380 and 690 nm. In a 30-min integration HORuS gets a S/N ~ 1500, 250, 35 at 500 nm per resolution element for a V=7, 11, 15 mag solar-like star, respectively. A rudimentary exposure time calculator is available below. The grating and cross-dispersion prisms are fixed, and therefore so is the spectral coverage. In addition to the exposure time, only the CCD (4096x4096 Fairchild) binning can be chosen, with two recommended modes: no binning for very bright targets (V<9) where the readout time is not an issue (90 seconds), and 8x2 binning for fainter targets or for bright ones when the readout time needs to be minimized (5 seconds). A preliminary data reduction pipeline is available.