It will be shown that a
countable distributive lattice is projective if and only if the product of any two join
irreducible elements is join irreducible, and every element of the lattice is both a
finite sum of join irreducible elements and a finite product of meet irreducible
elements. For an arbitrary distributive lattice, necessary and sufficient conditions for
projectivity are obtained by adding to these conditions a further condition on the set
of join irreducible elements.