  We examine the electroweak breaking mechanism in the minimal supersymmetric
standard model (MSSM) using the {\em complete} one-loop effective potential
$V_1$. First, we study what is the region of the whole MSSM parameter space
(i.e. $M_{1/2},m_o,\mu,...$) that leads to a succesful $SU(2)\times U(1)$
breaking with an acceptable top quark mass. In doing this it is observed that
all the one-loop corrections to $V_1$ (even the apparently small ones) must be
taken into account in order to get reliable results. We find that the allowed
region of parameters is considerably enhanced with respect to former "improved"
tree level results. Next, we study the fine-tuning problem associated with the
high sensitivity of $M_Z$ to $h_t$ (the top Yukawa coupling). Again, we find
that this fine-tuning is appreciably smaller once the one-loop effects are
considered than in previous tree level calculations. Finally, we explore the
ambiguities and limitations of the ordinary criterion to estimate the degree of
fine-tuning. As a result of all this, the upper bounds on the MSSM parameters,
and hence on the supersymmetric masses, are substantially raised, thus
increasing the consistency between supersymmetry and observation.
