Circular RNAs (circRNAs) are a type of non-coding "RNA molecules that lack a 5'-terminal cap and 3'terminal poly A tail. A large number of circRNAs have been identified through biological experiments, computational methods and high-throughput sequencing. CircRNA sequence composition determines if a given circRNA is exonic, intronic or retained-intronic. CircRNAs are more abundant and stable than linear mRNAs, and their expression is both step-and location-specific. CircRNAs mediate transcriptional and post -transcriptional regulation of gene and protein expression. CircRNAs regulate cancer development via multiple mechanisms, including miRNA sponges, modulating Wnt signaling pathway and epithelial mesenchymal transition. An in-depth study of circRNA will provide a better understanding of carcinogenesis and assist in developing clinical diagnostic and therapeutic strategies. (C) 2017 Elsevier B.V. All rights reserved.