Publishers may only allow sharing of certain versions of a manuscript: the preprint (before peer review), the post-print (the peer reviewed, accepted, unformatted manuscript), or the publisher's version (the final, formatted manuscript).
Normally, the pre-print version can be self-archived (shared on institutional repository), but the post-print and publisher's version normally can't be shared. Look up the publisher on SHERPA Romeo to learn which versions of your work can be shared online or archived in a repository. Authors can also add addenda to publishing agreements in order to make their work available in an open access or institutional repository.
