The role of chemoradiotherapy within the realm of rectal cancer treatment is rapidly expanding as we enter an era of organ preservation and deferral of surgery. As such, there is an urgency to predict which tumours are likely to completely respond, and a need for real-time response monitoring to enable personalised treatment. The accurate measurement of tumour hypoxia could fill prognostic and response monitoring voids. It is projected that DRS can be used to measure colorectal tumour hypoxia in a murine xenograft model to predict and reflect a response to radiotherapy. If proven, this could effect changes in future clinical practice through incorporation in endoscopic or implantable devices for hypoxia sensing for advanced prognostication and response monitoring – and this work begins by engaging with healthcare professionals and rectal cancer patients to ascertain applicability and acceptability of suggested devices to guide future device development. This research interrogates subcutaneous colorectal tumours with a Diffuse Reflectance Spectroscopy (DRS). This thesis first seeks to validate DRS for the assessment tumour vascular oxygenation (TVO) through assessment of carbogen inhalation effect, and through comparison with the OxyLite Sensor – the present gold standard for the assessment of cellular hypoxia. Findings suggest that the two modalities correlate well in a non-tumour bearing flank, but the correlation is lost when assessing the tumour. The natural course of TVO is then characterised, and it is demonstrated that it negatively correlates with both time and tumour volume, although there is high degree of heterogeneity. However, the correlation is of sufficient strength to enable a machine learning algorithm to accurately predict tumour size based on the DRS oxygenation readout. Further experiments introduce radiotherapy into the fold, and it is demonstrated in a subset of mice that a statistically significant enhancement in TVO occurs on days 5 and 7 post radiotherapy.