After reading your post (I had the same problem) I tried every descendant of java.util.Collection. It worked only with java.util:collection itself. the nice thing is that without specifying anything, JBoss created a link table between the two entities. Which is what should be done. example: /** * @return the roles */ @ManyToMany( targetEntity=Role.class, cascade={CascadeType.PERSIST, CascadeType.MERGE} ) public Collection < Role > getRoles() { if(roles == null) { roles = new LinkedList < Role >(); } return roles; } /** * this setter must exist for hibernate but be private because we don't want anyone to use it * @param roles the roles to set */ @SuppressWarnings("unused") private void setRoles(Collection < Role > roles) { this.roles = roles; } When the Roles are read from the database, the Class used for the roles Collection is org.hibernate.collection.PersistentBag It would actually be nice to be able to choose a type that disallows duplicates in the list. Because now I have to catch the exception upon "persist". View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4128070#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...