  We introduce the notion of a hamiltonian 2-form on a Kaehler manifold and
obtain a complete local classification. This notion appears to play a pivotal
role in several aspects of Kaehler geometry. In particular, on any Kaehler
manifold with co-closed Bochner tensor, the (suitably normalized) Ricci form is
hamiltonian, and this leads to an explicit description of these Kaehler
metrics, which we call weakly Bochner-flat.
  Hamiltonian 2-forms draw attention to a general construction of Kaehler
metrics which interpolates between toric geometry and Calabi-like constructions
of metrics on (projective) line bundles. They also arise on
conformally-Einstein Kaehler manifolds. We explore these connections and
ramifications.
