In many cases, you will need to obtain permission from a rights holder or pay licensing fees in order to make use of in-copyright material. Some reasons you may want to get permission include:
The Association of Research Libraries Code of Best Practices in Fair Use offers this nugget of wisdom:
"Exercising fair use is a right, not an obligation. There will always be situations in which those entitled to employ fair use may forgo use or obtain permission instead. . . But the choice to seek a license or ask permission should be an informed one."
Numerous institutions have developed useful materials to help you navigate the often complex process of obtaining permission for the use of copyrighted materials.