  The aim of this paper is to define and study Yetter-Drinfeld modules over
Hom-bialgebras, a generalized version of bialgebras obtained by modifying the
algebra and coalgebra structures by a homomorphism. Yetter-Drinfeld modules
over a Hom-bialgebra with bijective structure map provide solutions of the
Hom-Yang-Baxter equation. The category $_H^H{\mathcal YD}$ of Yetter-Drinfeld
modules with bijective structure maps over a Hom-bialgebra H with bijective
structure map can be organized, in two different ways, as a quasi-braided
pre-tensor category. If H is quasitriangular (respectively coquasitriangular)
the first (respectively second) quasi-braided pre-tensor category
$_H^H{\mathcal YD}$ contains, as a quasi-braided pre-tensor subcategory, the
category of modules (respectively comodules) with bijective structure maps over
H.
