We calculate the electronic thermal conductivity in a d-wave supercond
uctor, including both the effect of impurity scattering and inelastic
scattering by antiferromagnetic spin fluctuations. We argue that phono
ns dominate heat transport near T-c, but that electrons are responsibl
e for most of the peak observed in clean samples, The peak position is
predicted to vary nonmonotonically with disorder, in good agreement w
ith experiments on YBa2(Cu1-xZnx)(3)O-7-delta.