In the case of material that is quite old, the material might be in the public domain and no longer protected by copyright. Because copyright law in the U.S. has changed a lot over the years, it can be complicated to figure out if a work is still protected by copyright. See Peter Hirtle's (Cornell) excellent public domain chart or read more about the public domain at the Columbia copyright website.
Other materials, such as those produced by the federal government or those licensed with Creative Commons licenses, are also available for use without permission. Watch this brief video to learn more about Creative Commons licenses.
Sometimes the rights information provided can be confusing or misleading. If you're unsure whether the materials you want to use are copyrighted or not, there are a couple of things you can do: