In situ breast carcinoma represents an early localized stage in the transition to invasive breast carcinoma and has an especially favorable prognosis with appropriate management. The widespread use of mammography has contributed to its increased rate of diagnosis. The two histologic forms of this entity have distinct pathologic and biologic characteristics, with different therapeutic implications. Lobular carcinoma in situ is considered a marker of increased risk for subsequent invasive carcinoma and is most commonly managed by nonoperative surveillance. Ductal carcinoma in situ behaves more as a true anatomic precursor of invasive disease and has the same therapeutic options as invasive breast carcinoma. Minimally invasive breast carcinoma should be considered a fundamentally different entity, primarily because of its potential for systemic metastasis that in situ lesions do not theoretically have. There are still many outstanding issues and dilemmas to be resolved by scientific investigation before the intriguing potential of these early forms of breast malignancy are fully understood.