Advances in surgical resection, development of adjuvant therapies and efforts to optimise early detection, have led to a reduction in colorectal cancer mortality rates, but despite this progress around 40% of patients still die from recurrence or metastatic disease. Poor response to adjuvant treatment for colorectal cancer (CRC) continues to pose a significant problem. The cancer stem cell hypothesis suggests that within tumours a small population of cells, colorectal Cancer Stem Cells (CSC), are the driving force behind carcinogenesis: triggering gene transcription; maintaining tumour growth and ultimately fuelling resistance to chemo-radiation and instigating disease recurrence. The identification of reliable CSC markers, including Leucine-rich repeat containing G-protein coupled Receptor-5 (Lgr5), has facilitated further investigation. Lgr5 is a Wnt target gene, fundamental for the growth and maintenance of intestinal stem cells and deregulation of the Wnt / β3-Catenin pathway; it is implicated in most sporadic colorectal cancers. Further, the NFĸB co-factor BCL-3 has been shown to increase the expression of Lgr5 in colorectal tumour cells. Interestingly, co I ore ctaI cancers with increased expression of BCL-3 are associated with a worse prognosis. The Cyclo-oxygenase-2 / Prostaglandin-E2 (COX-2 / PGE2) pathway has also been shown to stimulate the function and survival of colorectal CSCs through up-regulating Lgr5 expression. The role of Aspirin and other Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) (that inhibit COX) in supressing colorectal carcinogenesis is well established in both high- high risk patient groups and in those who have already been treated for CRC. Rather than administering Aspirin as a prophylaxis, this project aimed to utilise a combined approach of in-vitro and in-vivo translational research, to investigate its role as an adjunct to chemo-radiotherapy in CRC, specifically exploring the COX-2 / PGE2 and BCL-3 pathways in conferring resistance to chemo-radiation and how this might be ameliorated with Aspirin treatment. Aspirin significantly enhanced the response to radiation in colorectal carcinoma cell lines, but not in adenoma derived cells. Compared to the effect of radiation alone, the combination of radiation and 2mM Aspirin significantly increased apoptosis and reduced cell proliferation, conferring a two-fold reduction in survival. Further, analysis of the role of low-dose, long-term Aspirin treatment revealed that colorectal carcinoma cells could be sensitised to a lower dose of radiation, significantly reducing survival by an additional 30%. Investigation of potential mechanisms of action demonstrated suppression of markers associated with cancer stem cells in some adenoma and carcinoma cell lines, supporting the hypothesis that through inhibiting COX-2 and thus lowering PGE2 levels, Aspirin represses the function and survival of Lgr5+ CSCs. Further, in HCA7/Par carcinoma line, expression of COX-2 and BCL-3 was repressed following Aspirin treatment compared to radiation alone and a greater reduction was seen with the combination of treatments, suggesting that Aspirin exerts its radio-sensitising effect on Lgr5+ cells by targeting two important pro-survival pathways; COX-2 / PGE2 and BCL-3/ NFKB. In parallel, a clinical, multi-centre, prospective cohort study (ASPIRE) was successfully established, aiming to explore Aspirin's role as an adjunct to preoperative chemo-radiotherapy in patients with rectal cancer. Ethical approval was granted until 2017 and as such the study is still recruiting. Through the analysis of pathological outcomes and tissue, blood and urine specimens, it aims to investigate whether Aspirin can enhance tumour regression in response to neo-adjuvant radiation and to investigate potential biomarkers of response. We anticipate that ASPIRE will translate our findings in the laboratory and hence will justify a novel clinical application for Aspirin in colorectal cancer; providing targeted treatment to patients, improving response to therapy and thus reducing local recurrence and increasing overall survival.