This book addresses the pressing problem of drug resistance in the treatment of breast cancer. It provides a comprehensive understanding of the complex mechanisms underlying drug resistance and presents the latest advances in therapeutic approaches to overcome this enormous challenge. The book begins with an introduction to the biology of breast cancer, treatment modalities and the mechanisms of drug resistance. This includes the principles of drug resistance in targeted, immune-, endocrine and chemotherapy. In the following sections, the authors present innovative strategies to overcome drug resistance and new research directions, among others the role of the microbiome in drug resistance, molecular profiling of patients for personalized medical care, and new drug delivery systems. Finally, the book concludes with a discussion of the differences in the prevalence and treatment success of breast cancer between high- and low-income countries. This book is an informative resource for students, researchers, and clinicians in the field of breast cancer research and oncology in general.