The relationship between pregnancy and breast cancer is complex, and a paucity of available data further complicates decision-making for many women diagnosed with breast cancer during pregnancy or desiring to become pregnant after such a diagnosis. Treatment of breast cancer during pregnancy requires a multidisciplinary care team and careful consideration of the risk of the disease and gestational age of the fetus, in conjunction with the patient's preferences. Chemotherapy should be deferred beyond the first trimester. There is no evidence that pregnancy in a breast cancer survivor will decrease long-term survival; in fact, studies suggest a potential protective effect of pregnancy after breast cancer in terms of the risk of recurrence. However, the available studies are limited by substantial potential biases, and concerns remain for some women and their doctors about the risks of pregnancy after breast cancer. This article reviews what is known about the association between pregnancy and breast cancer, discusses treatment options for women diagnosed with the disease during pregnancy, and summarizes evidence regarding the safety of pregnancy after breast cancer.