We develop a method for the construction of the effective potential at high
temperatures based on the effective field theory approach and renormalization
group. It allows one to sum up the leading logarithms in all orders of
perturbation theory. The method reproduces the known one-loop and two-loop
results in a very simple and economic way and clarifies the issue of the
convergence of the perturbation theory. We also discuss the assumptions being
made for the determination of the critical temperature of the electroweak phase
transition, and analyse different perturbative uncertainties in its
determination. These results are then used for the non-perturbative lattice
Monte Carlo simulations of the EW phase transition in forthcoming paper.Comment: 44 pages, preprint CERN-TH.6973/9