  In this note, an overview of Calabi-Yau varieties in positive characteristic
is presented. Although Calabi-Yau varieties in characteristic zero are
unobstructed, there are examples of Calabi-Yau threefolds in positive
characteristic which cannot be lifted to characteristic zero, although
one-dimensional and two-dimensional Calabi-Yau varieties, i.e., elliptic curves
and K3 surfaces, are all liftable to characteristic zero. In this respect,
Calabi-Yau threefolds in positive characteristic are interesting in view of
deformation theory and they are still very mysterious.
