The chapter focuses on quadratic-residue codes. The quadratic-residue (QR) codes L, image, N, image are cyclic codes of prime block length p over a field GF(l), where 1 is another prime which is a quadratic residue modulo p. These codes are defined and a summary of their properties is presented. Examples of quadratic-residue codes are the binary Hamming code, and the binary and ternary perfect Golay codes G23 and G11. QR codes have rate close to ½, and tend to have high minimum distance. Several techniques for decoding QR and other cyclic codes are described. The most powerful method is permutation decoding, which makes use of the fact that these codes have large automorphism groups. Other properties of QR codes discussed in this chapter are idempotents, dual codes, and extended codes of length p + 1, and automorphism groups. Further properties of binary QR codes are described including methods for finding the true minimum distance.