Seeing statistics from several year-long site-testing campaigns carried out at different locations at the Roque de los Muchachos Observatory (ORM) are reviewed (this contribution contains results previously published by Munoz-Tunon et al. (1997) [Estudio de Sitio para el Gran Telescopio: Informe final; A&AS, 125, 183]). In particular, tests at the TNG, WHT, GTC1 and GTC2 sites are discussed. Routine measurements were taken with cross-calibrated differential image motion monitors. The values obtained for the two sites proposed for the Gran Telescopio de Canarias show excellent and similar results, with mean and median seeing values of 0.72 arcsec and 0.65 arcsec, consistent with those obtained in earlier campaigns at the ORM. Under typical seeing conditions, image quality does not depend on the particular location and shows a high degree of homogeneity over the whole Observatory. Differences may be more evident, up to 0.2 arcsec, for seeing poorer than 1.5 arcsec, which happens in less than 5/% of cases. Seeing better than 1/'' occurs in about 80/% of cases with an average of 25% meeting half-arcsecond conditions. This percentage can rise consistently to more than 50/% in the best months at all the locations tested. A dependence of seeing with the season of the year is definitively established with the best values during the summer period, which is found to be correlated with the height and thickness of the inversion layer. From the analysis of surface-layer meteorological data taken simultaneously at two of the sites a possible relationship is found between the wind properties (direction and speed) and seeing behaviour. Other meteorological parameters do not seem to play any significant role as far as image quality is concerned.