Background The epidemiology of cases of acute gastroenteritis (AGE) of viral etiology is a relevant pub- lic health issue. Due to underreporting, the study of outbreaks is an accepted approach to investigate their epidemiology. The objective of this study was to investigate the epidemio- logical characteristics of AGE outbreaks due to norovirus (NoV) and sapovirus (SV) in Catalonia. Material and Methods Prospective study of AGE outbreaks of possible viral etiology notified during two years in Catalonia. NoV and SV were detected by real time reverse transcription polymerase (RT- PCR). Results A total of 101 outbreaks were registered affecting a total of 2756 persons and 12 hospitali- zations (hospitalization rate: 0.8x1,000,000 persons-year); 49.5% of outbreaks were food- borne, 45.5% person to person and 5% waterborne. The distribution of outbreaks according to the setting showed a predominance of catering services (39.6%), nursing homes and long term care facilities (26.8%) and schools (11.9%). The median number of cases per out- break was 17 (range 2 - 191). The total Incidence rate (IR) was 18.3 per 100,000 persons- years (95%CI: 17.6 - 19.0). The highest IR was in persons aged 65 years (43.6x100,000 (95% CI: 41.0 - 46.2)) (p < 0.001). A total of 1065 samples were analyzed with a positivity rate of 60.8%. 98% of positive samples were NoV (GII 56.3%; GI 4.2%; GII+GI 4.2%; non- typ- able 33.0%). SV was identified in two person-to-person transmission outbreaks in children. Conclusions These results confirm the relevance of viral AGE outbreaks, both foodborne and person-to- person, especially in institutionalized persons. SV should be taken into account when inves- tigating viral AGE outbreaks.