  High-precision calculations of hadron spectroscopy are a crucial task for
Lattice QCD. State-of-the-art techniques are needed to disentangle the
contributions from different energy states, such as solving the generalized
eigenvalue problem (GEVP) for zero-momentum hadron correlators in an efficient
way. We review the method and discuss its application in the determination of
the $B_s$-meson spectrum using (quenched) nonperturbative HQET at order
$1/m_b$.
