  Let X be a normal complex projective variety with at worst klt singularities,
and L a big line bundle on X. We use valuations to study the log canonical
threshold of L, as well as another invariant, the stability threshold. The
latter generalizes a notion by Fujita and Odaka, and can be used to
characterize when a Q-Fano variety is K-semistable or uniformly K-stable. It
can also be used to generalize volume bounds due to Fujita and Liu. The two
thresholds can be written as infima of certain functionals on the space of
valuations on X. When L is ample, we prove that these infima are attained. In
the toric case, toric valuations acheive these infima, and we obtain simple
expressions for the two thresholds in terms of the moment polytope of L.
