Breast cancer is the most common cancer in young women (aged 40 years or younger), with over 2,000 new cases each year in the United Kingdom. Younger women have been found to develop more aggressive tumours coupled with lower survival and higher local-recurrence rates compared to older women. The exact reasons for this remain unclear, and evidence that age is an independent factor for poor prognosis still remains limited. There is further need for more in-depth research into this area to help inform both patients and the clinical teams treating these patients. The aim of this thesis was to study factors affecting outcomes for young women with breast cancer to provide additional data for clinicians and their patients, to weigh up the optimum approach to reduce the risk of death in newly diagnosed young breast cancer patients. This thesis includes research comprising a collection of published works in young women with invasive breast cancer, using data from a large prospective cohort study. The findings have demonstrated that the oestrogen receptor status of the tumour, together with the ethnicity and body mass index of patients, were found to be significant independent prognostic factors affecting survival in this young age group, whilst reported family history of breast cancer, surgical type and BRCA mutation status were not found to be significant prognostic indicators. There is a need for caution when extrapolating data from older patient cohorts in order to determine the most appropriate treatment management options for younger women. Future research should be carried out in order to investigate new treatment approaches for this age group, and should take into account these prognostic factors to provide clinicians with sufficient information to decide the optimum treatment approach to reduce the rate of death for young women with invasive breast cancer.