Communication-network planning is a complex process involving a number of interrelated factors among which transmission performance is of major importance. In the paper, detailed consideration is given to the various aspects of transmission performance in relation to speech, telegraph, programme and television channels.Under the auspices of the international consultative organisations (CCITT and CCIR) much study has been devoted to the subject of performance planning. The rapidly increasing length and complexity of modern communication channels have intensified this study in recent years. Several of the major recommendations of the CCITT and CCIR are enumerated and discussed in the paper.Transmission-performance assessment, particularly of speech channels, is a fundamental factor in determining transmission performance limits. The paper includes brief information on assessment studies.The provisions of the new United Kingdom national transmission plan and of the proposed new CCITT switching and transmission plan for worldwide telephony circuits are defined and discussed in some detail.