The article by Erika Lasda and Roy Parker reviews the diversity and functions of circular RNAs (circRNAs) in biological systems. CircRNAs can be produced through various mechanisms, including direct ligation of linear RNA ends, RNA processing reactions, and backsplicing. They exhibit unique properties such as the potential for rolling circle amplification, rearrangement of genomic information, resistance to exonucleases, and constraints on RNA folding. CircRNAs have been identified in various organisms, including plants, animals, and archaea, and are involved in multiple biological processes. The authors discuss the biogenesis, metabolism, and functions of circRNAs, highlighting their roles as templates for viroid and viral replication, regulators of transcription, snoRNAs, and miRNA sponges. They also explore the identification and validation methods for circRNAs, their cellular localization, and potential roles in disease. The article concludes by emphasizing the need for further research to understand the biological functions of circRNAs and their implications in disease.The article by Erika Lasda and Roy Parker reviews the diversity and functions of circular RNAs (circRNAs) in biological systems. CircRNAs can be produced through various mechanisms, including direct ligation of linear RNA ends, RNA processing reactions, and backsplicing. They exhibit unique properties such as the potential for rolling circle amplification, rearrangement of genomic information, resistance to exonucleases, and constraints on RNA folding. CircRNAs have been identified in various organisms, including plants, animals, and archaea, and are involved in multiple biological processes. The authors discuss the biogenesis, metabolism, and functions of circRNAs, highlighting their roles as templates for viroid and viral replication, regulators of transcription, snoRNAs, and miRNA sponges. They also explore the identification and validation methods for circRNAs, their cellular localization, and potential roles in disease. The article concludes by emphasizing the need for further research to understand the biological functions of circRNAs and their implications in disease.