Abstract: Circular RNAs (circRNAs) are a novel type of RNA that function as microRNA (miRNA) sponges, RNA-binding proteins, and regulators of nuclear transcription. CircRNAs play a dual role in cancer, wherein their function as miRNA sponges could lead to either an oncogenic stimulus or tumor suppression. CircRNAs can also serve as novel diagnostic and prognostic biomarkers, as well as therapeutic targets for cancer; both these functions have great potential for clinical application in cancer diagnosis and therapy. Here, we review the mechanisms of action and potential clinical applications of circRNAs in tumors.