Investigations on the vaccination against the Bluetongue Virus serotypes 4 and 8 in sheep The aim of this cumulative thesis was to investigate on the one hand whether vaccinations with inactivated Bluetongue Virus (BTV) 8 vaccines induce antibodies in sheep still detectable after five to seven and a half years. On the other hand different vaccination schemes and techniques in sheep using inactivated BTV 4 and BTV 8 vaccines were compared under field conditions. In the first study the presence of BTV antibodies was proven five to seven and a half year after previous vaccinations with inactived BTV 8 vaccines. Boosted vaccinations improved seropositivity but even after only one vaccination antibodies were found after seven and a half years. In the second study eight different vaccination schemes using four commercially available inactivated Bluetongue vaccines against serotypes 4 and 8 were tested in three different combinations (setting 1 3) under field conditions for their ability to generate a measurable immune response in 240 sheep. All vaccine combinations were well tolerated by all sheep. In setting 1 there were no relevant significant differences in the immunogenic response between simultaneously applied vaccines at different sites or at the same injection site with a twin syringe. Importantly a one shot vaccination induced comparable immunogenicity to a boosted injection. Low serotype specific neutralising antibody levels were detected in settings 2 and 3 and are attributed to diverse factors which may have influenced the measured immunogenicity due to the setting as a field trial. The results of the described studies propose that vaccination recommendations could be simplified substantially to motivate more farmers to vaccinate their ruminant livestock.