2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/98098We 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.LaTeX, 41 pages; updated to improve exposition and coherence with part II (math.DG/0401320)Differential GeometryAlgebraic Geometry53C55; 53C25, 53B35text