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Platelet activation, thrombus growth and subsequent thromboembolism underpins the pathophysiology of ischaemic stroke. At sites of atherothrombotic plaque rupture, it is established that platelet surface receptor glycoprotein (GP) VI-dimer binds to exposed sub-endothelial collagen, initiating the signalling required to cause platelet activation. Then, separate platelet-fibrin interactions cause the thrombus to grow, culminating in distal tissue ischaemia; crucial in thrombotic diseases such as ischaemic stroke. The work in this thesis demonstrates that GPVI-dimer binds to fibrin also, causing platelet activation through a mechanism independent of collagen. In ischaemic stroke this is important as it not only implicates GPVI-dimer in the large-artery atherosclerotic stroke subtype, it suggests a key role in cardioembolic stroke, where fibrin, rather than collagen, is the key platelet ligand. We compared the platelet surface expression of GPVI-dimer in a control, pre-stroke atrial fibrillation (AF), and a stroke population admitted to hospital using flow cytometry. We also measured platelet activation by platelet P-selectin exposure in all the cohorts. The results demonstrate that both AF and stroke patients have more ‘active’ circulating platelets compared to the controls. Furthermore, both AF and stroke patients – irrespective of AF type or stroke aetiology, express more GPVI-dimer on their platelets compared to controls. In the stroke cohort, GPVI-dimer expression was significantly higher at day-90 post-stroke than at admission. The ability of GPVI to interact with the two main ligands that drive thrombosis, collagen and fibrin, cements its role as a key platelet receptor in human thromboembolic disease. These results intimate an important role for GPVI-dimer in driving thrombotic risk pre-stroke, as well as after having a stroke, suggesting that the direct inhibition of GPVI-dimer could be a promising future antithrombotic target.