  The construction of an effective 3D theory at high temperatures for the MSSM
as a model of electroweak baryogenesis is discussed. The analysis for a single
light scalar field shows, that given the experimental constraints, there is no
value of the Higgs mass for which a sufficiently first-order phase transition
is obtained. A precise determination of the 3D parameters of the effective
theory for the case of a light right-handed stop allows us to obtain an upper
bound on the masses of the lightest Higgs and right-handed stop using the
two-loop effective potential. A two-stage phase transition persists for a small
range of values of $m_{\tilde{t}_{R}}$.
