Human herpes virus 6 (HHV6) infects the majority of individuals in childhood, followed by a lifelong asymptomatic latent infection. However, in immunosuppressed individuals reactivation of HHV6 can cause significant clinical pathology. Recent successes with adoptive T cell therapy against other viral infections, notably the human herpes viruses Epstein-Barr virus (EBV) and Human cytomegalovirus (HCMV), suggest that this may be a useful therapeutic approach for HHV6-driven disease in immunosuppressed individuals. However, very few studies have been carried out analysing the immune response to HHV6 in any detail. This thesis was aimed at characterising the CD8+ T cell response to HHV6 in a group of healthy individuals, with the aim of mapping and characterising novel CD8+ T cell epitopes. Initial studies included four HHV6B antigens (U11, U39, U54 and U90), predicted to be immunogenic based on their HCMV homologues. Whole antigen peptide mixes (pepmixes) were used to stimulate peripheral blood mononuclear cells (PBMC) from healthy subjects. T cell responses were analysed by intracellular cytokine staining (ICS) after overnight stimulation and/or by interferon-γ (IFN-γ) ELISpot assay after 10 days of stimulation. For responses to U11 and U90, peptides libraries were used to map minimum CD8+ restricted epitopes. Further characterisation of HHV6B-specific T cells was carried out by identifying the HLA restriction elements and determining whether these T cells were capable of killing HHV6B-infected cells. PBMC from 30 healthy donors were stimulated with pepmixes corresponding to HHV-6B antigens U11, U39, U54 and U90. A weak CD8+ response (0.02-0.2%) to U90 and U54 was observed in a number of donors. Short-term in-vitro reactivations of PBMC (in 25 healthy donors) with HHV6B pepmixes followed by analysis of antigen and peptide specific response were performed by IFN-γ ELISpot assay. T cell responses to U54, U90, U11 and U39 were observed in 88%, 84%, 76% and 72% of the donors, respectively. Subsequently, the breadth of epitope specificity within U90 and U11 was screened for 9 healthy donors; with successful identification of 10 CD8+ T cells specific (9-mer) epitopes within these antigens. Seven of them were within U90 antigens and three of them were within U11 antigens. Allelic association of the U90 epitopes were; VEESIKEIL - B40 (60), FESLLFPEL - B40 (60), NLITAAKNI - A2, ITAAKNIGI - A2, LNIDPSESI - A1, PSKSKKIKL - A29, NHCFINHFV - B39. Allelic association of the 2 U11 epitopes were LKTQRRHKF - B37 and GILDFGVKL - A2; the HLA association for FNAVYSQRV was not identified. CD8+ T cell populations specific to some of these epitopes were also able to kill HHV6B infected cells. HHV6B T cells responses are detectable in healthy donors. Peptide specific responses against U11 and U90 have been mapped and characterised. These findings are relevant to the development of T cell mediated immunotherapy of HHV6-associated diseases.