Abstract: Traditional concurrency control techniques for database systems (transaction management based on locking protocols) have been successful in many multiuser settings, but these techniques are inadequate in open, extensible and distributed hypertext systems supporting multiple collaborating users. The term "multiple collaborating users" covers a group setting in which two or more users are engaged in a shared task. Group members can work simultaneously in the same computing environment, use the same set of tools and share a network of hypertext objects. Hyperbase (hypertext database) systems must provide special support for collaborative work, requiring adjustments and extensions to normal concurrency control techniques. Based on the experiences of two collaborative hypertext authoring systems, this paper identifies and discusses six concurrency control requirements that distinguish collaborative hypertext systems from multiuser hypertext systems. Approaches to the major issues (locking, notification control and transaction management) are examined from a supporting technologies point of view. Finally, we discuss how existing hyperbase systems fare with respect to the identified set of requirements. Many of the issues discussed in the paper are not limited to hypertext sytsems and apply to other collaborative systems as well.