  In this paper, unstable metric entropy, unstable topological entropy and
unstable pressure for partially hyperbolic endomorphisms are introduced and
investigated. A version of Shannon-McMillan-Breiman Theorem is established, and
a variational principle is formulated, which gives a relationship between
unstable metric entropy and unstable pressure (unstable topological entropy).
As an application of the variational principle, some results on the
$u$-equilibrium states are given.
