Breast cancer is the commonest form of cancer in New Zealand women. Most cancers are removed either with breast conserving surgery (BCS) or mastectomy. With BCS, it is important to remove all, or virtually all, of the tumour to minimise the risk of local recurrence. The quality of evidence on how much margin of normal tissue needs to be obtained from around the cancer to minimise risk of recurrence is poor. As a result, about one in five women currently undergo a second operation – either a wider excision or mastectomy. This results in additional stress, side effects and costs for these women. This study will examine the excision margins in more than 10,000 women in order to determine what is the optimal margin of clearance to minimise risk recurrence and need for additional surgery, and how does this differ by patient factors, cancer type, and other treatments.