  In this article, we provide a detailed survey of techniques for hexahedral
mesh generation. We cover the whole spectrum of alternative approaches to mesh
generation, as well as post processing algorithms for connectivity editing and
mesh optimization. For each technique, we highlight capabilities and
limitations, also pointing out the associated unsolved challenges. Recent
relaxed approaches, aiming to generate not pure-hex but hex-dominant meshes,
are also discussed. The required background, pertaining to geometrical as well
as combinatorial aspects, is introduced along the way.
