Breast cancers are highly variable with regard to pathological and clinical features. While various different cancers being at different stages of tumor progression might explain some of this variability, we must also consider evidence that breast cancers can also develop along different molecular pathways. This manuscript discusses the extensive heterogeneity of breast cancer at the pathological and molecular level, reviewing evidence for a breast cancer progression model as well as evidence that breast cancer is not a single disease. In addition to this heterogeneity among different breast cancers, there is often pathological and molecular heterogeneity among different areas within individual neoplasms. This heterogeneity within cancers probably reflects genetic instability, and could also explain the ability of breast cancers to adapt to new environmental situations. Understanding this heterogeneity, among different breast cancers as well as within individual cancers, is important for understanding the complexity of this disease and ultimately for managing breast cancer effectively.