This program was performed to evaluate the effects of load history variations on the durability of composite structures in order to outline procedures for developing load spectra for design and test. A total of 36 constant amplitude and 184 spectrum fatigue tests were performed. The test specimen was a simple center hole coupon made of AS/3501-6 graphite/epoxy prepregs. Two different layups, fiber dominated (48/48/4) and matrix dominated (16/80/4), were evaluated. The spectrum variations found to have greatest effect were those that increased the number or magnitude of the high loads in the spectrum. Addition of overloads caused more than an order of magnitude reduction in life. A 5 percent increase in test limit stress caused about 60 percent decrease in life. Variations found to have smaller impact on life were those that changed the lesser loads in the spectrum.