The liability of a digital signal to error is one of the most significant transmission impairments experienced in a digital communication network. Consideration is given to the activities and status of studies within the International Telegraph and Telephone Consultative Committee (CCITT). The sources of interference causing errors are discussed and an introduction is given to Hypothetical Reference Connections (HRX). The internationally agreed end-to-end error performance specification for a digital connection is reviewed along with the methodology for allocating this overall objective to constituent transmission equipments operating over different media. A number of mathematical models are identified which might be adequately representative of the error characteristics to be found in real networks. Finally, measurement aspects are discussed.