  We construct Hopf bimodules and Yetter-Drinfeld modules of Hopf algebroids as
a generalization of the theory for Hopf algebras. More precisely, we show that
the categories of Hopf bimodules and Yetter-Drinfeld modules over a Hopf
algebroid are equivalent (pre-)braided monoidal categories. Moreover, we also
study the duality between finitely generated projective Yetter-Drinfeld
modules.
