Objective: The objective of this article is to describe the hormonal pathways required for breast development in cisgender women and to review the current available literature describing breast growth and breast cancer risk in transgender women. Methods: Literature review and discussion. Results: Early mammary tissue development occurs prenatally. This process is hormone-independent and occurs similarly in males and females. Breast tissue is quiescent until puberty, at which time surging estrogen levels in cisgender girls mediate breast development and growth. Adult breast tissue composition further evolves in cisgender women during pregnancy, lactation, and menopause, revealing the ever-changing interplay between breast structure and hormonal environment. Conclusion: Breast growth is a significant physical endpoint in the hormonal treatment of transgender women. Transgender hormone regimens, which typically pair an estrogen with an anti-androgen, can help achieve this goal.