Objectives: Bluetongue (BT) and epizootic hemorrhagic disease (EHD) viruses are two important vector-borne orbiviruses with a worldwide distribution that have major impacts on livestock breeding and trade. They are transmitted by hematophagous Culicoides species. If epidemiological investigations in epizootic territories conducted after massive outbreaks enabled to better understand the epidemiology of these diseases, their epidemiology remain poorly studied/understood in areas of endemicity. In this context, three studies were implemented in Reunion Island located in the South Western Indian Ocean Materials and Methods: The three studies were: (i) a virological and serological survey on cattle to identify BT and EHD viruses seasonal dynamics and prevalence, (ii) an entomological study on Culicoides species diversity, seasonal dynamics and spatial distribution, (iii) a viral screening in Culicoides species to identify and better characterize the vector species involved in the viral transmission, the level of infection in field populations and virus dynamics in Culicoides populations. Results: The period with highest levels of seroconversion occurred is the hot and rainy season, when incidence of antibody acquisition reached 36.7% (IC95% = [31.7%; 41.7%]) for EHD virus and 20.7% (IC95% = [16.3% ; 25.1%]) for BT virus. Five Culicoides species are described on the island, and seasonal dynamics is particularly marked for three of them. Four species are involved in the transmission of each virus. Conclusion: Overall, BT is endemic with continuous virus circulation, limited clinical cases and high serological prevalence while EHD is epizootic with a marked seasonal transmission pattern.