In order to know the present importance of grass weeds infestations in winter cereal crops in Catalonia, a survey in all the areas of the region has been done. Grass weeds infestations have been observed in 61.4% of the surveyed fields, which means that almost 200,000 ha of cereal are infested by these species. Among the most important because of frecuence and infestation level, we can mention: Lolium rigidum Gaud., Avena sterilis L. subsp. ludoviciana (Durieu) Nyman, Bromus diandrus Roth, Avena fatua L. and Avena sterilis L. subsp. sterilis. Lolium rigidum and Avena sterilis subsp. ludoviciana have been found in more than 50% of the surveyed fields in both dryland and irrigated conditions. These species mean more than 75% of the field infestations by weeds. The distribution of Avena fatua is restricted to the areas with a cooler and wetter climate; while Avena sterilis subsp. sterilis is present in those regions with mild winters. Finally Bromus diandrus has been found in dryland continental regions.