Disclosed herein are circular RNAs and transfer vehicles, along with related compositions and methods of treatment. The circular RNAs can comprise group I intron fragments, spacers, an IRES, duplex forming regions, and/or an expression sequence, thereby having the features of improved expression, functional stability, low immunogenicity, ease of manufacturing, and/or extended half-life compared to linear RNA. Pharmaceutical compositions comprising such circular RNAs and transfer vehicles are particularly suitable for efficient protein expression in immune cells in vivo. Also disclosed are precursor RNAs and materials useful in producing the precursor or circular RNAs, which have improved circularization efficiency and/or are compatible with effective circular RNA purification methods.