  We reexamine the strength of the first order phase transition in the
electroweak theory supplemented by an extra Higgs doublet. The
finite-temperature effective potential, $V_{eff}$, is computed to one-loop
order, including the summation of ring diagrams, to study the ratio
$\phi_c/T_c$ of the Higgs field VEV to the critical temperature. We make a
number of improvements over previous treatments, including a consistent
treatment of Goldstone bosons in $V_{eff}$, an accurate analytic approximation
to $V_{eff}$ valid for any mass-to-temperature ratios, and use of the
experimentally measured top quark mass. For two-Higgs doublet models, we
identify a significant region of parameter space where $\phi_c/T_c$ is large
enough for electroweak baryogenesis, and we argue that this identification
should persist even at higher orders in perturbation theory. In the case of the
minimal supersymmetric standard model, our results indicate that the extra
Higgs bosons have little effect on the strength of the phase transition.
