The vast majority of breast conditions that are biopsied (sampled) are benign [1]. Some of these benign conditions are associated with the increased risk for the subsequent development of cancer and some are not 1–5 Those that are associated with an elevated risk can be placed into a spectrum of changes referred to as proliferative breast disease (with or without atypia). They are considered marker lesions in contrast to precursor lesions, which will be described in Chap. 4. Marker lesions are associated with risk for cancers in both breasts. If cancer develops in the same breast as a marker lesion, it is likely to arise at a location in that breast other than where the marker lesion was found, in contrast to precursor lesions, for which subsequent cancers are usually found in close proximity to where the precursor lesion was located.