Circular RNAs are a novel class of non-coding RNA characterized by the presence of a covalent bond linking the 3′ and 5′ ends generated by backsplicing. Circular RNAs are widely expressed in a tissue and developmental-stage specific pattern and a subset displays conservation across species. Functional circRNAs have been shown to act as cytoplasmic microRNA sponges and RNA-binding protein sequestering agents as well as nuclear transcriptional regulators, illustrating the relevance of circular RNAs as participants in the regulatory networks governing gene expression. Here, we review the features that characterize circular RNAs, discuss putative circular RNA biogenesis pathways as well as review the uncovered functions of circular RNAs. This article is part of a Special Issue entitled: Clues to long noncoding RNA taxonomy1, edited by Dr. Tetsuro Hirose and Dr. Shinichi Nakagawa.