This thesis focused on the long-term mental health and quality of life of breast cancer survivors, compared to women with no prior cancer. The first study was a systematic review of studies that assessed adverse mental health outcomes in women who had breast cancer and non-cancer controls. This found evidence suggestive of an increased risk of anxiety, depression, suicide, and neurocognitive and sexual dysfunctions in breast cancer survivors. The second study systematically summarised the lists of Read codes and clinical definitions used in previous studies of mental health-related outcomes in primary care databases of electronic health records in the UK. The results showed substantial heterogeneity across studies and informed the definition of the outcomes in this thesis. The third study used data from the UK Clinical Practice Research Datalink (CPRD) GOLD primary care database to quantify the risk of adverse mental health-related outcomes in 57,571 breast cancer survivors and 230,067 women with no previous cancer. Breast cancer survivorship was positively associated with anxiety, depression, fatigue, pain, sexual dysfunction, sleep disorder and being prescribed opioid analgesics, but there was no evidence of association with cognitive dysfunction or fatal and non-fatal self-harm. The fourth study included 353 breast cancer survivors and 252 women with no prior cancer who replied to questionnaires assessing quality of life and mental health. Compared to women with no prior cancer, breast cancer survivors had poorer quality of life in the domains of cognitive problems, sexual function, and fatigue, but no evidence of difference in negative feelings, positive feelings, pain, or social avoidance. Women with advanced-stage cancer at diagnosis, and/or prior receipt of chemotherapy, had poorer quality of life and mental health. In conclusion, breast cancer survivorship is associated with impaired quality of life and raised risk of adverse mental health-related outcomes, persisting well into the survivorship period.