Glioblastoma (GBM) is the most common aggressive primary brain tumour in adults. Despite various recent treatment advances, prognosis and survival rates remain dismal. A potential explanation for such poor outcomes is the high invasive nature of glioma cells, which enable them to invade areas of the brain where the blood-brain barrier may still be intact. This could explain the frequently encountered tumour recurrences that occur at the edges of the original tumour or at the surgical resection margins. Another potential explanation for such poor outcomes is the blood-brain barrier (BBB), which prevents delivery of effective chemotherapy into the brain. Furthermore, efflux transporters at the BBB can also restrict drug delivery. P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two efflux transporters that work closely together as efflux pumps for a variety of anticancer drugs. Temozolomide (TMZ) is the central chemotherapy agent used in the treatment of malignant brain tumours but interestingly, its relationship with P-gp and BCRP remains poorly understood. The ultimate aim of this thesis is to develop an imaging method for measuring the effects of strategies modifying Temozolomide (TMZ) delivery into the brain and brain tumours. An in-vitro BBB model was developed and validated. This was then used to conduct TMZ transport studies, which showed a trend for TMZ to be transported by P-gp and BCRP at clinically relevant concentrations. In order to confirm these results, dynamic small animal positron emission tomography (PET) with [11C]TMZ was also performed in wild-type and in mice lacking P-gp, BCRP or both transporters as well as in wild-type mice treated with the efflux transporter inhibitor Tariquidar (TQD). Scans confirmed higher delivery of [11C]TMZ in mice lacking P-gp, BCRP and both transporters. Similar results were noted in wild type mice following the pharmacological inhibition of P-gp and BCRP. A human glioma model in mice was derived by intracranial injection of human U87 glioma cells in thirty-five athymic female mice. Animals were treated with TMZ alone, TQD alone or combination therapy (TMZ and TQD). Response to treatment was evaluated by performing volumetric tumour measurement using MRI. Combination therapy was shown to be superior achieving significant early and sustained response in tumours when compared with TMZ alone treatment. Finally, PET imaging was used with the known P-gp substrate (R)-[11C]verapamil to demonstrate the functional activity of P-gp in five high-grade glioma patients. This approach was shown to be a suitable non-invasive imaging method for visualising P-gp function in patients with brain tumours and highlighted the heterogeneity of the functional activity of P-gp in these patients.