  We perform a nonperturbative study of the electroweak phase transition (EWPT)
in the two Higgs doublet model (2HDM) by deriving a dimensionally reduced
high-temperature effective theory for the model, and matching to known results
for the phase diagram of the effective theory. We find regions of the parameter
space where the theory exhibits a first-order phase transition. In particular,
our findings are consistent with previous perturbative results suggesting that
the primary signature of a first-order EWPT in the 2HDM is $m_{A_0} > m_{H_0} +
m_Z$.
