This thesis addresses the hypothesis that multiple sclerosis (MS) patients have an imbalance in the frequencies of natural killer cell subsets, with fewer immunoregulatory and more proinflammatory NK cells compared to healthy controls. Ten NK cell subsets were defined according to expression of CD56, CD8, CD27 and CD57. Subset frequencies were compared by performing multiparameter flow cytometry on PBMCs from healthy controls and patients with clinically isolated syndrome (CIS) or relapsing-remitting MS. CD56dim CD8+ CD57+ CD27- NK cells had a higher frequency in untreated relapsing-remitting MS patients compared to healthy controls and their frequency was decreased in patients on interferon-β treatment. CD56bright NK cells, particularly the CD8- CD27- subset, had a significantly lower frequency in untreated MS and CIS patients. CD8- CD57- CD27- subsets of both CD56dim and CD56bright NK cells were expanded in interferon-β-treated patients in remission but not relapse. Characteristics of two CD56dim subsets displaying an apparent reciprocal relationship (CD8+ CD57+ CD27- and CD8- CD57- CD27-) were compared by qRT-PCR, flow cytometry and killing assays. CD56dim CD8+ CD57+ CD27- NK cells were more cytotoxic and expressed higher levels of transcripts encoding IFNγ, T-bet and other factors associated with IFNγ signalling. This is consistent with the hypothesis that there is a shift towards a more pro-inflammatory NK cell phenotype in MS patients, which is reversed by interferon-β treatment. In order to determine whether these NK cells exert their effects mainly in the blood or in the central nervous system (CNS), mononuclear cells were extracted from the CNS of Line 7 mice, which develop spontaneous autoimmune demyelination due to expression of a human MS-associated HLA/TCR combination. NK cells were detected in spleen, but not CNS, of these mice, suggesting that their role in this particular disease model is restricted to regulation of other leukocytes outside the CNS.