  For a finite distributive lattice $D$, let us call $Q \subseteq D$
\emph{principal congruence representable}, if there is a finite lattice $L$
such that the congruence lattice of $L$ is isomorphic to $D$ and the principal
congruences of $L$ correspond to $Q$ under this isomorphism.
  We find a necessary condition for representability by principal congruences
and prove that for finite distributive lattices with a join-irreducible unit
element this condition is also sufficient.
