More than 20% of colorectal cancers are diagnosed following an emergency presentation in England, which is associated with poor survival. Little is known on the clinical circumstances surrounding emergency presentations and on the complex contribution of demographic, clinical and tumour factors. Such information is crucial to develop effective strategies for reducing emergency presentations, particularly among higher risk groups. In this thesis, I reviewed the literature on the role of comorbidities in influencing timely cancer diagnoses and used an epidemiological population-based approach to profile variation in risk of emergency presentations. Specifically, I examined the type and timing of symptoms, comorbidities and benign diagnoses prospectively recorded during the months or years precancer diagnosis using individually linked cancer registration, primary care and secondary care data on nearly 9,000 colorectal cancers diagnosed in England 2005-2010. The project revealed that emergency presenters have similar ‘background’ consultation history as non-emergency presenters. Their tumours seem associated with less typical symptoms, however one fifth of emergency presenters had consulted with typical alarm symptoms indicating opportunities for earlier diagnosis. Patients with proximal colon cancer had a higher risk of emergency presentations, despite having more frequent consultations with relevant symptoms, highlighting that tumour factors contribute to emergency presentations. ‘Serious’ comorbidities (diabetes, cardiac, respiratory diseases) diagnosed/treated in secondary care were associated with emergency cancer diagnosis, possibly because they might have distracted doctors and patients from prompt cancer investigations. The risk of emergency presentation was greater for women aged 40-59 years with gynaecological or recently diagnosed benign intestinal conditions, which might have provided alternative explanations. In conclusion, this thesis has contributed to the understanding of the distinct influence of patient (age, sex, comorbidities) and tumour/disease factors (type and timing of symptoms and tumour sub-sites) on the risk of emergency presentations, highlighting potential responsible mechanisms that can be targeted by future interventions. Greater integration between primary and secondary care, multidisciplinary diagnostic centers and novel technologies, such as quantitative faecal haemoglobin testing (FIT), might help to seize the opportunities for earlier diagnosis and reduce emergency presentations.