Interest in prognostic factors for breast cancer has been stimulated by the success of systemic adjuvant therapy for early-stage operable disease. Patients destined for recurrence can be selected for systemic adjuvant therapy, while patients not likely to recur can be spared the morbidity of unnecessary treatment. The number of tumor-related features available for prognosis has grown impressively in recent years. The purpose of this article is to review tumor-related biologic factors and relate them to prognosis and treatment objectives.