A review is made of the different cumulative damage theories available in the literature.A new approach (unified theory) is suggested which can be applied to stress-controlled or strain-controlled conditions, which considers the order of application of different stress or strain levels.Comparison is made with a large number of test results using several different levels.The theory is also applied to some cases of random loading.It is found that the proposed theory yields an improved agreement with experimental results especially for cases where there is a large difference between levels.