Despite intensive work on molecular carcinogenic mechanisms and novel drug development, cancer has still remained an incurable disease. With the hope to gain therapeutic advantage or miracle cure, more than 50% of cancer patients consume various kinds of herbal supplements when undergoing chemotherapeutic treatment with antineoplastic agents. However, numerous preclinical studies on combined treatment of various cancer cells with natural dietary flavonoids and chemotherapeutic drugs have revealed both desired (additive to synergistic) as well as undesired (antagonistic) interactions, meaning that coadministration of flavonoids and chemotherapeutic drugs can lead to both augmentation as well as abolishment of therapeutic efficacy. This book is the first one to systematically compile currently available information about the modulation of chemotherapeutic efficacy by flavonoids, hopefully being an assistant for cancer patients to make conscious choices in selecting proper plant products during chemotherapeutic treatment.