OBJECTIVES: To review the data available on excision margins following breast-conserving therapy (BCT), focusing on definitions of positive and clear margins, percentage of operations resulting in positive margins, the effect of positive margins on future treatment, and the relationship between positive margins and disease-free and overall survival.  METHODS: Targeted searches of PubMed were conducted using a predefined search strategy. Data from robust systematic reviews and/or meta-analyses were given priority.  RESULTS: Definitions of positive and negative margins are variable, but typically a clear margin of 2 mm is considered acceptable. Most studies indicate positive margins in 20%-40% of patients after wide local excision. Guidelines recommend that patients with positive margins after BCT undergo repeat surgery, and in surveys, most physicians said they would recommend re-excision when there is tumour within 1 mm of the margin. In the identified studies, 20%-30% of patients underwent re-excision and approximately 2% had multiple re-excisions (two or more); 10%-15% of patients who initially had lumpectomy later had a mastectomy. There is a significant association between margin status and local recurrence (in a recent meta-analysis, the odds ratio was 2.42 for positive vs. negative margin status; 95% confidence interval, 1.94-3.02; P<0.001). However, among patients with a clear margin, width is not clearly related to risk of local recurrence. Four studies that assessed the effect of margin status on overall or disease-specific survival were identified, three reported a significant association (e.g., cause-specific survival at 12 years significantly associated with margin status, P<0.001).  CONCLUSIONS:  Definition of adequate margins remains controversial. None-the-less, final margin status is a key prognostic factor following BCT. The data identified suggest that an intervention that reduces the rates of positive margins during BCT may have the potential to improve outcomes and reduce the burden on patients and health care providers.