A Killing $p$-form on a Riemannian manifold is a $p$-form whose covariant derivative is totally anti-symmetric. In this paper we give the complete (local) description of 4-dimensional Riemannian manifolds (M,g) carrying non-parallel Killing 2-forms $��$. If $M$ is connected and oriented, we show that there exists a dense open subset of $M$ on which one of the three exclusive situations holds: either $��$ is everywhere degenerate and $g$ is conformal to a product metric, or $g$ is conformal to an ambik��hler metric obtained via the Calabi construction from a polarized Riemannian surface, or $g$ is conformal to an ambitoric structure of hyperbolic type, and depends locally on two functions of one variable. We also give compact examples, by constructing infinite-dimensional families of Riemannian metrics carrying Killing 2-forms of each of the above types on $S^4$ and on Hirzebruch surfaces.

36 pages