Hopf algebroids and quantum groupoids

We introduce the notion of Hopf algebroids, in which neither the total algebras nor the base algebras are required to be commutative. We give a class of Hopf algebroids associated to module algebras of the Drinfeld doubles of Hopf algebras when the R-matrices act properly. When this construction is applied to quantum groups, we get examples of quantum groupoids, which have Poisson groupoids as their semi-classical limits. The example of quantum sl(2) is worked out in details.