Hi Tatum, We do allow some highlighting in our rentals but we ask that you dont highlight excessively. If you return the rental back to us with over one-third of the text highlighted, we may have to mark the book as damaged. Highlighting can be distracting for future renters and we may not be able to add an overly-highlighted book back into our inventory. Consider using page flags to mark an important sentence. Flags can help you easily get back to the page you need. Page flags are also small enough that they can be used multiple times on the same page. Plus, they are really easy to remove from the textbook before you send it back at the end of the semester. Let us know if you have any other questions. Thank you!