  We present a detailed analysis of the phase transition in the standard model
at finite temperature. Using an improved perturbation theory, where plasma
masses are determined from a set of one-loop gap equations, we evaluate the
effective potential $V_{eff}(\varphi,T)$ in next-to-leading order, i.e.,
including terms cubic in the gauge coupling $g$, the scalar self-coupling
$\lambda^{1/2}$ and the top-quark Yukawa coupling $f_t$. The gap equations
yield a non-vanishing magnetic plasma mass for the gauge bosons, originating
from the non-abelian self-interactions. We discuss in detail size and origin of
higher order effects and conclude that the phase transition is weakly
first-order up to Higgs masses of about $70\ GeV$, above which our calculation
is no longer self-consistent. For larger Higgs masses even an approximation
containing all $g^4$ contributions to $V_{eff}$ is not sufficient, at least a
full calculation to order $g^6$ is needed. These results turn out to be rather
insensitive to the top-quark mass in the range $m_t=100\ -\ 180\ GeV$. Using
Langer's theory of metastability we calculate the nucleation rate of critical
droplets and discuss some aspects of the cosmological electroweak phase
transition.
