This book is a comprehensive guide to breast toxicity. Adjuvant radiation remains standard for a majority of women who undergo breast-conserving surgery for breast cancer, and indications for post-mastectomy and regional lymph node irradiation have also broadened with recent publications. At the same time, locoregional recurrence has declined and survival has improved in recent decades. In the current era of excellent breast cancer outcomes, then, considering the balance between toxicity and outcomes becomes paramount. Several recent editorials recommend considering toxicity against the potential benefit of adjuvant radiation in tailoring radiation decisions for individual patients. Thus, a clear understanding of the potential toxicities of adjuvant radiation for