Abstract: Circular RNAs (circRNAs) are a recently recognized group of RNA molecules with a closed loop structure, lacking a 5′ cap and a 3′ poly(A) tail. Once thought to be splicing errors, they are now known for their stability, conservation, and specific expression in different cell types. Research has shown that circRNAs play important roles in cancer development and progression. This review summarizes current knowledge about circRNA formation, properties, and functions in cancer. We describe how circRNAs can act as oncogenes or tumor suppressors by sponging microRNAs, interacting with RNA-binding proteins, regulating transcription, or being translated into peptides. We also list examples of circRNAs altered in cancers such as glioblastoma, lung, breast, gastric, and colorectal, and explain how they affect key cancer traits like uncontrolled growth, evading growth suppression, spreading, and forming new blood vessels. In addition, we discuss the strong potential of circRNAs as clinical tools. Because they are stable and found in body fluids like blood and saliva, circRNAs could serve as non-invasive biomarkers for early cancer detection, prognosis, and monitoring treatment response. Finally, we consider targeting harmful circRNAs or restoring beneficial ones as new cancer therapies, and outline the challenges and future directions in this fast-moving field.