Summary: Efficient use of block codes to communicate over a telemetry channel is dependent upon the knowledge of the instants, in time, that one code waveform, or code word, ends and the succeeding waveform begins. This word synchronization is the subject of concern in this paper. Two basically different single-channel synchronization techniques are discussed. The first relies upon the identification of periodically transmitted prefixes, which unavoidably increase the redundancy of the code without increasing its synchronous reliability. The second method eliminates the need for prefixes entirely and relies instead upon certain properties of the code itself. Both these techniques are investigated more closely as applied to binary orthogonal codes. Asymptotic results are obtained which show that the number of received words necessary to obtain synchronization with a fixed prefix length increases in proportion to the size of the code when the prefix method is used. In contrast, the number of words which need to be observed for the prefix-free codes remains constant independent of the code length. Finally, some results of the search for prefix-free codes of moderate size are reported.