  We use the quasiclassical theory of superconductivity to calculate the
electronic contribution to the thermal conductivity. The theory is formulated
for low temperatures when heat transport is limited by electron scattering from
random defects and for superconductors with nodes in the order parameter. We
show that certain eigenvalues of the thermal conductivity tensor are universal
at low temperature, $k_B T\ll \gamma$, where $\gamma$ is the bandwidth of
impurity bound states in the superconducting phase. The components of the
electrical and thermal conductivity also obey a Wiedemann-Franz law with the
Lorenz ratio, $L(T)=\kappa/\sigma T$, given by the Sommerfeld value of
$L_{\!S}=({\pi^2}/{3})(k_B/e)^2$ for $k_BT\ll\gamma$. For intermediate
temperatures the Lorenz ratio deviates significantly from $L_{\!S}$, and is
strongly dependent on the scattering cross section, and qualitatively different
for resonant vs.\ nonresonant scattering. We include comparisons with other
theoretical calculations and the thermal conductivity data for the high $T_c$
cuprate and heavy fermion superconductors.
