Peripheral T-cell lymphomas (PTCLs) are rare diseases with a high mortality that respond poorly to conventional chemotherapy. They represent, therefore, a relatively large unmet medical need. Gene expression profiling has shown that a large subgroup of PTCL are derived from normal follicular helper (Tfh) T-cells. Interleukin-2 inducible T-cell kinase (ITK) is a tyrosine kinase and member of the TEC family of kinases, of which Bruton's tyrosine kinase (BTK), a B-cell signalling protein is also a member, and is expressed in Tfh cells and is important for T-cell receptor (TCR) and chemokine signalling. ITK is highly expressed in Tfh derived PTCL and is, therefore a potential target for treatment. It is inhibited by the clinically active BTK inhibitor, ibrutinib. We have compared siRNA knockdown of ITK and small molecule ITK inhibitors (ITKi) for their effects on proliferation and signalling in human T-cell lines and normal human primary T-cells. There was variation in some effects but all inhibitors repressed proliferation, calcium signalling and cytokine production and caused modest increases in apoptosis. Studies of mice with homozygous disruption of the Itk locus, by other researchers, showed that ITK deficiency skewed CD4 T-cell subset differentiation from Th17 to Treg but Tfh differentiation was not investigated. We were able to show that the effects previously reported in vivo were reproduced on tonsillar CD4 T-cells cultured under polarising conditions in vitro by ITKi. Unexpectedly, we showed that ITK inhibitors prevented Tfh differentiation whilst promoting that of suppressive Treg cells. Blocking IL-21, an important Tfh cytokine regulated by ITK, with IL21R-Fc partially prevented ITKi effects on Tfh and Treg differentiation suggesting a mechanism of action. We also had the opportunity to investigate two cases of primary human T-cell lymphoma and showed that the lymphoma cells had the capacity to differentiate to Tfh cells in vitro, which was prevented by ITKi, accompanied by an increase in Tregs. This suggested the exciting hypothesis that ITK inhibitors might alter the T-cell lymphoma environment from activating Tfh to inhibitory Treg. We speculate that in addition to preventing proliferation and enhancing apoptosis, a third potential therapeutic mechanism for ITK inhibitors in PTCL is skewing the microenvironment towards repression. Finally, in an effort to show that ITK inhibitors can be a component of combination treatments we demonstrated synergy with chemotherapy and PI3K inhibitors. The data presented in this thesis supports ITK as a target for therapy of Tfh-derived T-cell lymphomas.