Circular RNAs (circRNAs) are a class of non-coding RNAs in cells that lack a 5-terminal cap and a 3-terminal poly-A tail. These covalently closed RNA molecules are made in large numbers in eukaryotes and exhibit various roles in epigenetically changing cell functions. Recent original studies have demonstrated that the deregulated profile of circular RNA expression could promote cancers via different mechanisms, including changes in growth signals, proliferation potency of cells, angiogenesis, and invasion of tumoral cells. CircRNAs could act as oncogenic or tumor-suppressive agents under certain circumstances, and an increasing number of studies have identified these up-/down-regulated circRNAs in cancer. Moreover, researchers have introduced the potential of these RNAs to be predictive and prognostic biomarkers. Cancers are a significant cause of death across all ages worldwide and introducing new biomarkers, therapeutic targets, and prognostic factors could help reduce their mortality and morbidity. This section aims to introduce circRNAs as modulators of epigenetics in cells and explain their functions. It will also be demonstrated how these changes could lead to cancer formation and progression and how circRNAs could serve as potential novel biomarkers of cancer.