Sometimes it is difficult or impossible to identify the copyright holder for a particular work. It may be possible to identify who owned the rights at some point in time, but after a creator's death, or after the dissolution of an organization or company the trail of ownership may disappear. These works are referred to as "orphan works" - and there is no person who can give permission to use them. But that doesn’t mean they can’t be used.
You can accept the risk that a copyright holder may later identify herself and object to a use (a risk that is lessened when the work is potentially in the public domain, or when the use is potentially fair), or give up the plan of using such a work. If you choose to use an orphan work, make sure you provide a simple notice about how to assert a claim of ownership, such as "PLEASE NOTE: If anyone feels that this work is not a fair use of any materials, please write to _______________ at ________________ [email address]. The authors/creators will seriously consider such concerns and make every effort to respond appropriately."
Orphan works are a real problem in copyright law. Legislation has been proposed several times that would address some of the problems, but it has never been passed.
(Based on materials graciously made available by the University of Minnesota Libraries)