CircRNAs are a novel class of noncoding RNAs characterized by a covalently closed loop structure lacking both a 3′ polyadenylation tail and a 5′ cap. Increasing evidence demonstrated that circRNAs can participate in several biological processes including immune system physiology and immune-related diseases. CircRNAs act on immune pathways through several molecular mechanisms including miRNAs and RBP sponges, protein scaffolds, and mRNA stability modulation via diverse signaling pathways including MAPK, JAK-STAT, and Wnt signaling pathway. Recently, circRNAs have also been studied as biomarkers for immune diseases as well as new potential therapeutic targets. Effects of dysregulated circRNA levels correlate, among others, with alterations in immune cell behavior, cytokines production, and immune response in general. In this chapter, we describe new experimental progress in circRNAs that participates in the regulation of the immune system and in the pathogenesis of autoimmune diseases and infectious diseases.