Circular RNAs are a class of long noncoding RNA that were recently rediscovered as diverse, highly abundant, conserved and naturally occurring RNAs in eukaryotes. They are characterized by their 5′ and 3′ covalently joined ends. Some studies have attributed functions for circular RNAs, such as miRNAs sponges and transcriptional regulators, indicating that they may be largely biomarkers of both physiological and pathological processes. Circular RNAs have the potential to play important roles in transcription and post-transcription, giving rise to a whole complexity level to gene expression regulation. In this review, we discuss the biogenesis of circular RNAs, their properties and functions as well as different methods for their identification and their role in some diseases.