Glioblastoma multiforme (GBM) is an aggressive malignant brain tumour with a median survival for adults of only 11 – 15 months. Published studies have shown that prognosis can be improved if near-complete removal of the tumour can be achieved in surgery [do we need to put references?]. To aid resection, fluorescence guidance is used, utilising blue light excitation of a red fluorescing photosensitiser to highlight diseased cells. There is a need to better understand the light-tissue interactions during fluorescence guided resection, investigating the depth in tissue information present in the returned image and whether photodynamic therapy can be successfully performed with the existing photosensitiser. Building upon the knowledge accumulated over 13 years of collaboration on light interaction with skin, Monte Carlo Radiative Transfer will be used to investigate these clinical challenges in the brain and to continue our research on the impact of light on skin for both therapy and public health.