  We study renormalization-group flows in Yukawa theories with massless
fermions, including determination of fixed points and curves that separate
regions of different flow behavior. We assess the reliability of perturbative
calculations for various values of Yukawa coupling $y$ and quartic scalar
coupling $\lambda$ by comparing the properties of flows obtained with the beta
functions of these couplings calculated to different orders in the loop
expansion. The results provide a determination of the region in $y$ and
$\lambda$ where calculations up to two loops can yield reasonably reliable
results.
