Most journal publishers now allow authors to deposit their papers in repositories, but different journals have different rules. If you are unsure about a particular journal, email digitalcommons@longwood.edu and we'll help you figure it out.
Some allow inclusion in a repository before (pre-print) or after (post-print) the paper's publication. Some have an embargo period between publication and deposit in a repository. Some will also stipulate how post-prints should be formatted. The SHERPA project website at gives details of publisher's current policies on self-archiving and copyright.