Circular RNAs (circRNAs) are a diverse class of RNAs with varying sizes, cellular abundance, and biological functions. Investigations from the past decade have revealed that circRNAs are ubiquitously found in eukaryotes and have defined the different biological roles of circRNAs to illuminate this previously unrecognized class of molecules. In the context of the immune system, immune responses and immune-related diseases alter circRNA expression. More recently, several oncogenic double-stranded DNA viruses have been found to encode circRNAs. In this review, we summarize the current understanding of circRNAs and their emerging functions in immune regulation and autoimmune disorders, and discuss the identification and potential roles of viral circRNAs during infections. Finally, we present promising areas for future investigations in the nascent field of circRNAs. Circular RNAs (circRNAs) are a recently identified class of ubiquitously expressed RNAs found in eukaryotes, archaea, and viruses.DNA viruses Kaposi's sarcoma-associated herpesvirus, Epstein–Barr virus, and human papillomavirus encode circRNAs. Some circRNAs possess pro-viral functions as miRNA sponges and templates for translation.CircRNAs are differentially expressed in immune cells and in autoimmune diseases, including rheumatoid arthritis and systemic lupus erythematosus.Emerging data suggest that circRNAs have biological roles in the immune system as transcriptional regulators, protein sponges, and templates for translation.