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Chemokines are responsible for directing leukocyte migration and triggering firm arrest by activating integrins on leukocytes. It is now apparent that chemokines have critical biological roles beyond chemo-attraction. Throughout this thesis, I describe the importance of the CCL5/CCR5 axis in the context of the immune response and cancer biology. Specifically, CCL5 invokes dose-dependent distinct signalling events downstream of CCR5 activation in T cells. I show that nM concentrations of CCL5 mediate CD4+ T cell migration that is partially dependent on mTOR activation. CCL5 induces phosphorylation and de-activation of the repressor 4E-BP1, resulting in its dissociation from the eukaryotic initiation factor-4E to initiate protein translation. I provide evidence that CCL5 initiates rapid translation of cyclin D1 and MMP-9, known mediators of cell migration. The data demonstrated that up-regulation of chemotaxis-related proteins may “prime” T cells for efficient migration. During an immune response, recently recruited T cells are exposed to high CCL5 concentrations. The propensity of CCL5 to form higher-order aggregates at high, µM concentrations, prompted studies to investigate their effects on T cell function. I show that at these high doses, CCL5 induces apoptosis in PM1.CCR5 and MOLT4.CCR5 T cell lines. CCL5-induced cell death involves the cytosolic release of cytochrome c and caspase-9/-3 activation. Furthermore, I identified Tyrosine-339 as a critical residue within CCR5, suggesting that tyrosine phosphorylation signalling events are important in CCL5-mediated apoptosis. Our data suggest that CCL5-induced cell death, in addition to Fas/FasL mediated events, may contribute to clonal deletion of T cells during an immunological response. I subsequently examined the possible pathological consequence of aberrant CCL5/CCR5 signalling in breast cancer. Exogenous CCL5 enhances MCF-7.CCR5 proliferation, which is abolished by anti-CCR5 antibody and rapamycin. CCL5 induces the formation of the eIF4F translation initiation complex, and mediates a rapid up-regulation of cyclin D1, c-Myc and Dad-1 protein expression. Thus, our data demonstrate the potential for breast cancer cells to exploit downstream CCL5/CCR5 signalling pathways for their proliferative and survival advantage. Taken altogether, each of these studies reinforces the notion that chemokines are not only potent chemotactic mediators, but are key effectors in diverse developmental, immunological and pathological processes.