Breast cancers that occur in young women up to 5 to 10 years' postpartum are associated with an increased risk for metastasis and death compared with breast cancers diagnosed in young, premenopausal women during or outside pregnancy. Given the trend to delay childbearing, this frequency is expected to increase. The (immuno)biology of postpartum breast cancer is poorly understood and, hence, it is unknown why postpartum breast cancer has an enhanced risk for metastasis or how it should be effectively targeted for improved survival. The poorer prognosis of women diagnosed within 10 years of a completed pregnancy is most often contributed to the effects of mammary gland involution. We will discuss the most recent data and mechanistic insights of the most important processes associated with involution and their role in the adverse effects of a postpartum diagnosis. We will also look into the effect of lactation on breast cancer outcome after diagnosis. In addition, we will discuss the available treatment strategies that are currently being used to treat postpartum breast cancer, keeping in mind the importance of fertility preservation in this group of young women. These additional insights might offer potential therapeutic options for the improved treatment of women with this specific condition.