  We propose a generalization of Gysin maps for DM-type morphisms of stacks
$F\to G$ that admit a perfect relative obstruction theory $E_{F/G}^{\bullet}$,
which we call a "virtual pull-back". We prove functoriality properties of
virtual pull-backs. As applications, we analyze Gromov-Witten invariants of
blow-ups and projective bundles.
