The effect of uncertainties in the basic nuclear data needed in fission-product decay-heat summation calculations is considered. A variety of methods are developed to study the effect of errors in decay energies, half-lives, fission yields, and metastable states. Based on preliminary estimates of the uncertainties in the basic data, these methods show that decay heat for typical reactor exposures can be calculated with an accuracy of 7% or better for cooling times >10 sec. Attention is directed toward thermal fission of 235U, although the more general problem of other fissionable nuclides is considered. For cooling times <1000 sec, the major sources of error are due to uncertainties in the decay energies and fission-product charge distributions. All calculations are based on ENDF/B-IV cross sections.