The previous sections of this guide outlines many ways you might be able to legally use other's works without permission:
In all other cases, you must get permission from the copyright holder (i.e., obtain copyright clearance).
Include the following when you're seeking permission.
Keep track of your requests! If you do receive permission, it's important to maintain a record of this. If you do not receive permission, this may weigh in favor of fair use. If you send the request via email, save and file a PDF copy of your request and any response you receive. This PDF from the Copyright Office provides further suggestions for getting permission.
If you can't find contact information for a copyright holder, you may have encountered an orphan work. Unfortunately, this is quite common. Legislation has gone before Congress to deal with this problem, but so far there is no resolution. If you encounter an orphan work, here are some suggestions: