Circular RNAs (circRNAs) are a class of non-coding RNAs that do not have 5' end caps or 3' end poly (A) tails. There are more than one hundred thousand genes that encode circRNAs. Clinical data show that there are differences in the expression of circRNAs in a variety of diseases, including cancer, suggesting that circRNA has a regulatory effect on some diseases. Further studies reveal that circRNA can be used as an endogenous competitive RNA, thereby regulating the proliferation, invasion or other physiological activities of tumor cells. In addition, some circRNAs located in the nucleus can regulate the transcription of the parental gene by binding to RNA polymerase II. circRNA can also combine with proteins to influence the cell cycle. Furthermore, recent studies have shown that circRNA can encode proteins, similarly to mRNA. circRNAs are found extensively in human cells and have tissue specificity. They have the potential to be used in clinical applications as tumor markers and therapeutic targets.