  We consider liquid-vapor systems in finite volume $V\subset\R^d$ at parameter
values corresponding to phase coexistence and study droplet formation due to a
fixed excess $\delta N$ of particles above the ambient gas density. We identify
a dimensionless parameter $\Delta\sim(\delta N)^{(d+1)/d}/V$ and a
\textrm{universal} value $\Deltac=\Deltac(d)$, and show that a droplet of the
dense phase occurs whenever $\Delta>\Deltac$, while, for $\Delta<\Deltac$, the
excess is entirely absorbed into the gaseous background. When the droplet first
forms, it comprises a non-trivial, \textrm{universal} fraction of excess
particles. Similar reasoning applies to generic two-phase systems at phase
coexistence including solid/gas--where the ``droplet'' is crystalline--and
polymorphic systems. A sketch of a rigorous proof for the 2D Ising lattice gas
is presented; generalizations are discussed heuristically.
