Cancer is one of the leading causes of death worldwide. There are many problems in cancer therapy due to the side-effects which limit its usage. Products derived from natural substances, particularly polyphenolic compounds which have very little toxic effects on normal cells, have gained a crucial interest as therapeutic weapon in clinical oncology due to their chemopreventive, antitumoral, radiosensibilizing and chemosensibilizing activities against different types of aggressive, recurrent and drugresistant cancers.Especially now, polyphenols and their applications are one of the most studied topics in the literature due to their promising results against cancer cells. In many studies, it has been reported that, polyphenols inhibited the development of tumors through variety of mechanisms and reduce the tumor cell mass. However, with all the advantages of polyphenols, in the literature it is stated that, issues like poor solubility, high degradation rate and high dose requirement restrict the applications of polyphenols. Still, these obstacles can be overcome by using encapsulation and nano-drug delivery systems for plant-derived polyphenols. With the encapsulation techniques it is possible to increase their bioavailability, stability of the polyphenols and their uptake in the targeted cells. Encapsulated polyphenols have been used against cancer in various scientific studies. And the obtained results were promising. With the encapsulation methods, greater accumulation of polyphenols has been observed on the cell membrane and cytoplasm due to smaller size. In this chapter, plant-derived polyphenols, their stabilities and encapsulation of polyphenols in order to increase their stability and their potential as cancer treatment agents will be explained.