The paper considers group codes whose function is split between error correction and error detection with retransmission. For a given code, the minimum error probability is obtained when retransmission occurs whenever an error is detected. An estimate of the redundancy added by retransmission is given and the behavior of retransmission channels as the length of the code words increases is studied. Most of the analysis is for the binary symmetric channel, although some of the results apply to more general channels.