The concept of using digital computers for relaying originated some 25 years ago. Since then the field has grown rapidly. Computers have undergone a significant change - they have become more powerful, cheaper, and sturdier. Today computer relays are preferred for economic as well as technical reasons. These advances in computer hardware have been accompanied by analytical developments in the field of relaying. Through the participation of researchers at Universities and industrial organizations, the theory of power system protection has been placed on a mathematical basis. It is noted that, in most cases, the mathematical investigations have confirmed the fact that traditional relay designs have been optimum or near-optimum solutions to the relaying problem. This is reassuring: the theory and practice of relaying have been reaffirmed simultaneously.