AbstractInterleaving for error‐correcting codes is a method of composing a long‐burst‐error‐correcting code with combined short‐burst‐error‐correcting codes or random‐error‐correcting codes. The method is now employed in a wide variety of fields, such as communications, memory systems and PCM recording and is quite useful in increasing the reliability of such systems. However, there has been no detailed theoretical study of interleaving, although it has been the subject of numerous practical studies. A systematic study of its theoretical aspects and some new interleaving techniques are presented here. It is shown that, according to their construction, interleaved codes can be classified into the complete‐block type, convolutional type and incomplete‐block type, among which the incomplete‐block type is newly proposed. Other classifications of interleaving methods are made according to their types and numbers as well as their decoding methods. Next we review different trends in interleaving relevant to the types of codes to be used as the number of codes is changed. We then discuss an interleaving method with codes encoded by word units as well as an error‐detection code conduct a comparative investigation of the error‐correcting capability and encoding capability and encoding complexity as well as decoding from the viewpoint of construction. It is then shown that the incomplete‐block structure has an intermediate position between the complete‐block type and the convolutional type.