A new method for determining Beta, the reliability index, (and hence the probability of failure) of an earth retaining structure, has been evolved. The method employs a standard of mathematics no higher than that used in the deterministic evaluation of a factor of safety. Although the method has not been tested by any rigorous mathematic treatment it has been numerically checked by Monte Carlo simulations. Using the proposed method values of Beta have been obtained for several design problems. These values have been found to be very close to values of Beta obtained from Monte Carlo simulations of the problems (generally using not less than 10,000 iterations). Because of the simplicity of the method it is hoped that it will prove helpful to civil engineers interested in the idea of using probability theory to assist in their design work.