Circular RNAs (circRNAs) are a class of single-stranded, non-coding RNAs which form a covalently closed loop. It was first reported as viroids in plants and was later detected in human cells in 1979. In a circular RNA molecule, unlike a linear RNA, the 5' and 3' ends have been joined together. The unique loop structure of circRNAs provides them with a longer half-life and more resistance to RNase R than linear RNAs. CircRNAs are produced by non-canonical splicing process which is known as "backsplicing". In the process of backsplicing, a downstream splice donor is joined to an upstream splice acceptor. According to the sequence composition of circRNAs, they are categorized into various types, such as exonic circRNA, exon-intron circRNA (EIciRNA), and circular intronic RNA (ciRNA) etc.

Many studies have revealed that circRNAs have unique expression signatures and contribute to biological processes through a variety of functions, for example, acting as transcriptional regulators/microRNA(miRNA) sponges/protein templates, and serving as protein decoys, scaffolds and recruiters etc. The unique structure and broad expression of circRNAs makes them potential candidates for diagnostic biomarkers and therapeutic targets of a variety of diseases, such as cancers, cardiovascular diseases, neurological disorders and autoimmune diseases etc. More recently, an increasing number of investigations have been involved in the field of circRNA.