About 10% to 15% of patients who undergo breast-conservation surgery and radiation therapy will subsequently develop ipsilateral breast tumor recurrence (IBTR). This paper reviews the biology, clinical management, and outcome of this entity. Risk factors for IBTR include young age, positive microscopic margins, gross multifocality, an extensive intraductal component, and lymphatic vessel invasion. The standard therapy following IBTR has been mastectomy, but interest in further breast-conservation approaches has recently arisen. Although the outcome following salvage therapy is quite good, the risk of distant metastases for patients with IBTR is three to five times greater than for those without recurrence. The reason for this association has been controversial, but it now appears that IBTR is both a marker of the underlying biologic aggressiveness of the tumor and a source for further tumor metastasis. Monitoring of patients following lumpectomy and radiation therapy, and aggressive therapy for IBTR when diagnosed, are clearly warranted. Prognostic factors at the time of IBTR and implications for local and systemic therapy are discussed.