We describe the main features of the optical and mechanical design, and the architecture of the control system of SARG, the white pupil cross dispersed echelle spectrograph for the Italian Telescopio Nazionale Galileo (TNG) telescope. SARG is designed for the spectral range lambda equals 0.37 up to 0.9 micrometer, and for resolution from R equals 19,000 up to R equals 144,000. SARG uses an R4 echelle grating in quasi- Littrow mode; the beam size is 100 mm giving an RS product of RS equals 46,000 at order center. Cross-dispersion is provided by means of a selection of four grisms. A dioptric camera (F/5.05, R equals 144,000) images the cross dispersed spectra on a mosaic of two 2048 by 4096 EEV CCDs (pixel size: 13.5 micrometer). Expected peak efficiency is 0.17 at R equals 38,000, and greater than 0.10 over the whole range from lambda equals 0.4 to 0.9 micrometer. Confocal image slicers, modification of the Bowen-Walraven type designed by Diego, are foreseen for observations at R equals 76,000 (3 slices) and 144,000 (5 slices), allowing high efficiency even in fair seeing conditions. Minimum interorder separation is 8 arcsec. Further features of SARG include an absorbing cell for accurate radial velocities and a Lyot mask (located on an image of the entrance pupil before the slit) for spectrocoronographic observations. SARG is thermally controlled, in order to avoid deterioration of the optical performances.