A circular RNA encoding a polypeptide, wherein said circular RNA is single stranded and comprises a sequence of Formula II: [A n] -L 1 -[B o] Formula II wherein each A and B independently comprises any nucleoside; n and o are, independently 15 to 1000; and L 1 has the structure of Formula III: wherein a, b, c, d, e, and f are each, independently, 0 or 1; each of R 1, R 3, R 5, and R 7, is, independently, selected from optionally substituted C 1 -C 6 alkylene, optionally substituted C 1 -C 6 heteroalkylene, O, S, and NR 8; R 2 and R 6 are each, independently, selected from carbonyl, thiocarbonyl, sulfonyl, or phosphoryl; R 4 is optionally substituted C 1 -C 10 alkylene, optionally substituted C 2 -C 10 alkenylene, optionally substituted C 2 -C 10 alkynylene, optionally substituted C 2 -C 9 heterocyclylene, optionally substituted C 6 -C 12 arylene, optionally substituted C 2 -C 100 polyethylene glycolene, or optionally substituted C 1 -C 10 heteroalkylene, or a bond linking (R 1) a -(R 2) b -(R 3) c to (R 5) d -(R 6) e -(R 7) f, wherein if c, d, e, and f are 0, R 4 is not a bond; and R 8 is hydrogen, optionally substituted C 1 -C 4 alkyl, optionally substituted C 2 -C 4 alkenyl, optionally substituted C 2 -C 4 alkynyl, optionally substituted C 2 -C 6 heterocyclyl, optionally substituted C 6 -C 12 aryl, or optionally substituted C 1 -C 7 heteroalkyl; wherein L 1 is attached to [A n] and [B o] at the sugar of one of said nucleosides; and wherein said circular RNA comprises a coding region, a 5' untranslated region, a 3'UTR, and a poly-A tail, and wherein the poly-A tail terminates in the structure of Formula XXI: wherein N 3 is a nucleobase each of R 28, R 29, R 30, and R 31 is, independently, H, halo, hydroxy, thiol, optionally substituted C 1 -C 6 alkyl, optionally substituted C 1 -C 6 heteroalkyl, optionally substituted C 2 -C 6 heteroalkenyl, optionally substituted C 2 -C 6 heteroalkynyl, optionally substituted amino, azido, or optionally substituted C 6 -C 10 aryl; i is 0 or 1; X 5 is O, NH, or S; and X 6 is O or S; and X 7 is OH or SH, or a salt thereof. wherein each A and B independently comprises any nucleoside; n and o are, independently 15 to 1000; and L 1 has the structure of Formula III: wherein a, b, c, d, e, and f are each, independently, 0 or 1; each of R 1, R 3, R 5, and R 7, is, independently, selected from optionally substituted C 1 -C 6 alkylene, optionally substituted C 1 -C 6 heteroalkylene, O, S, and NR 8; R 2 and R 6 are each, independently, selected from carbonyl, thiocarbonyl, sulfonyl, or phosphoryl; R 4 is optionally substituted C 1 -C 10 alkylene, optionally substituted C 2 -C 10 alkenylene, optionally substituted C 2 -C 10 alkynylene, optionally substituted C 2 -C 9 heterocyclylene, optionally substituted C 6 -C 12 arylene, optionally substituted C 2 -C 100 polyethylene glycolene, or optionally substituted C 1 -C 10 heteroalkylene, or a bond linking (R 1) a -(R 2) b -(R 3) c to (R 5) d -(R 6) e -(R 7) f, wherein if c, d, e, and f are 0, R 4 is not a bond; and R 8 is hydrogen, optionally substituted C 1 -C 4 alkyl, optionally substituted C 2 -C 4 alkenyl, optionally substituted C 2 -C 4 alkynyl, optionally substituted C 2 -C 6 heterocyclyl, optionally substituted C 6 -C 12 aryl, or optionally substituted C 1 -C 7 heteroalkyl; wherein L 1 is attached to [A n] and [B o] at the sugar of one of said nucleosides; and wherein said circular RNA comprises a coding region, a 5' untranslated region, a 3'UTR, and a poly-A tail, and wherein the poly-A tail terminates in the structure of Formula XXI: wherein N 3 is a nucleobase each of R 28, R 29, R 30, and R 31 is, independently, H, halo, hydroxy, thiol, optionally substituted C 1 -C 6 alkyl, optionally substituted C 1 -C 6 heteroalkyl, optionally substituted C 2 -C 6 heteroalkenyl, optionally substituted C 2 -C 6 heteroalkynyl, optionally substituted amino, azido, or optionally substituted C 6 -C 10 aryl; i is 0 or 1; X 5 is O, NH, or S; and X 6 is O or S; and X 7 is OH or SH, or a salt thereof. wherein N 3 is a nucleobase each of R 28, R 29, R 30, and R 31 is, independently, H, halo, hydroxy, thiol, optionally substituted C 1 -C 6 alkyl, optionally substituted C 1 -C 6 heteroalkyl, optionally substituted C 2 -C 6 heteroalkenyl, optionally substituted C 2 -C 6 heteroalkynyl, optionally substituted amino, azido, or optionally substituted C 6 -C 10 aryl; i is 0 or 1; X 5 is O, NH, or S; and X 6 is O or S; and X 7 is OH or SH, or a salt thereof.