Semnani - shahid Beheshti University Tehran-Iran
Semnani - Afanced Technology Corporation U.S.A
Ehsan - Sharif University of Technoloy Tehran-Iran
Amini - Tehran Regional Electric Co. Tehran-Iran

A review of reliability indices as are related to distribution system design is given. Misinmization of total social cost is justified as the optimum approach for calculation of these indices. Diffierent techniques for estimating service interruption as part of the total social cost are also presented.