Cancer is a steadily increasing public health concern and severe socio-economic burden all over the world. Based on the current statistics of the WHO, almost 10 million people lost their lives due to malignant disorders in 2020, implying that one in every six deaths is due to cancer. In parallel, the global population of cancer survivors is also growing, propelled by advances in early detection, diagnosis and treatment methods, but also by demographic aging. More and more cancer patients today decide to use dietary supplements and herbal products in order to enhance their overall health, delay the progression of disease, augment the outcome of conventional therapies, mitigate diagnosis-related depression and anxiety, and alleviate drug-associated side effects, doing it mostly unbeknownst to their health care providers. Such over-the-counter supplements usually contain diverse flavonoids, such as catechins from green tea extracts, isoflavones from soy products, flavanones from citrus oils or silibinin from milk thistle seeds, just to name a few. A variety of flavonoid-rich products are often consumed concurrently with standard cancer therapies, without anticipating and understanding the potential interactions between plant-derived bioactive flavonoids and clinically used anticancer drugs. In this book, diverse effects of these polyphenolic phytochemicals on hormonal and targeted biological cancer therapies are covered, being a reasoned follow-up to the two previous books Plant Flavonoids Affect Cancer Chemotherapeutic Efficacy: A Handbook for Doctors and Patients (2019) and Dietary Flavonoids Interfere with Cancer Radiotherapy (2019). Besides naturally occurring flavonoids, the impact of flavonoid-rich plant extracts as well as semisynthetic derivatives on anticancer activities of co-administered hormonal and targeted drugs are thoroughly discussed, and some indicative recommendations to patients undergoing hormonal cancer therapies or targeted biological cancer therapies are provided.