XML (eXtensible Markup Language) has emerged as a relevant standard for document representation and exchange on the Web. It is often the case that XML documents contain information of different sensitivity degrees, which must be selectively shared by (possibly large) user communities. There is thus the need for models and mechanisms enabling the specification and enforcement of access control policies for XML documents. Mechanisms are also required enabling a secure and selective dissemination of documents to users, according to the authorizations that these users have. In this talk, we first define a model of access control policies for XML documents. Policies that can be defined in our model take into account both user profiles, and document contents and structures. We also describe an approach, which essentially allows one to send the same document to all users, and yet to enforce the stated access control policies. Our approach consists of encrypting different portions of the same document according to different encryption keys, and selectively distributing these keys to the various users according to the access control policies. We show that the number of encryption keys that have to be generated under our approach is minimal.