Margin width is an important factor influencing local recurrence risk for both ductal carcinoma in situ (DCIS) and invasive breast cancer treated with breast-conserving surgery. For DCIS treated with excision alone, a margin width of at least 2mm minimizes local recurrence risk. For DCIS treated with excision and radiation therapy, margins of 2mm or more are associated with reduced risk compared to narrower margins, but wider margins do not provide additional benefit. For invasive breast cancer, obtaining a negative margin, defined as no tumor cells on the inked edge, is associated with low recurrence rates and is recommended. Larger margins do not lower the risk further. Special cases may require individualized treatment plans including re-exc