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Failure rates have been collected at the Royal Radar Establishment since 1944 and continuous records of failures in radar equipments undergoing environmental testing have been maintained. The effect of environment on failure rates was investigated as early as 1945 and M.T.B.F's of 116 hours in England and Europe, 61 hours in the Mediterranean and 18 hours in the tropics were obtained in that year. Later analyses show that severe environments (heat, vibration, etc.) give a ratio of about 25:1 over the failure rates of temperature controlled, shock isolated equipment. The results of analyses of component part failures under six different operating environments are given. A table of present failure rates to be expected from groundbased equipment at maximum component ratings is included. The present position on the effect of de-rating on reliability is outlined. The mechanism of failure of component parts is examined first in general terms and then under three headings: high humidity, high temperature and low temperature. Examples are shown of the types of failure experienced under these environments. The effect due to each environment and the probable mechanism of failure is tabulated.