Get a good night's sleep, practice your 15 minutes. Be familiar with your material. Anticipate questions and prepare responses.

Listen to the feedback, don't take it personally, but do take it seriously.

If you don't have to start from scratch, you did a good job.

There are great responses here already, but three observations:

As others have said, you are the expert. My major professor told me that before my orals: no one had studied the specific subject of my thesis more than I and I was focused more than even the professors who would judge me. I approached the orals from a position of strength (and so will you). I like to suggestion that you sleep well the night before.

Your professors may argue among themselves. Let them. They will have some ego on the line, and they will need to show one another their own academic stuff, so that may result in their own internal disputes. Let them sort that out. Sit there quietly and let them do their thing. Do not become the soft wood for their chainsaws.

And finally: things could be worse. When my mentor, Sven S, Liljeblad (1899-2000), went for his orals, defending his 1927 dissertation on the Grateful Dead, he was required to publish and distribute his work throughout Europe. His major professor then invited the scholar he felt would most likely disagree with Sven to act as a prosecutor. Fortunately, Sven was allowed to invite the scholar he thought would most likely support him as a co-defender. The date of the orals was published internationally and everyone was encouraged to attend, turning it into something of an international conference. The orals lasted two days, and as Sven said, "and then all of Europe had fun at my expense." Your orals will likely be a little easier. People normally survive even the most grueling of orals, and you will too.

Best wishes, and break a leg (because, it is, after all, theatre).

Know the committee wants you to do well. And you know more about your topic than they do.

• Defend your viewpoint, but be humble. Realise that although you are well educated on your chosen topic there is still an awful lot you don't know. Admit that your interpretations are interpretations, and that these might well have to be reassessed if/when new evidence comes to light.

• Be prepared to engage in a discussion. Remember that historians as a rule like discussing, and that your thesis defense is not only about your thesis itself, but also about your ability to engage with questions and sources. Don't be afraid to ask questions of your opponents. If you are confronted with a statement you feel is incorrect or unfamiliar with, don't hesitate to ask things like "Why do you believe that?" or "How do you arrive at that conclusion?". Displaying informed curiosity is usually a big plus in any professional historian's book.

• Remember that you committee want you to do well as much as you do. Don't be afraid. You're among friends.

• Also remember that you probably know your material better than anyone else in the room. Be prepared to accept that there is a lot of stuff you don't know, or perhaps should have interpreted differently - but also don't be afraid to stand up for yourself. Explain why you think your explanation is correct, and how your sources brought you to this belief.

• Practice your fifteen minutes, and remember that time flies. Keep in mind that the committee has already read your thesis, and are usually interested in getting to the questions/discussion as quickly as possible. Keep your presentation short and to the point. In this case, less is usually more.

Good luck!

You know your stuff at this point, otherwise (I hope) your committee wouldn't have let you schedule your defense. There's no preparation you can do that will help, at this point, you've done it all in the process of researching, writing, and editing your thesis. You're the expert on your thesis topic, so act like it. If you really want something written to help keep your focused, a brief (1-2 pages) outline of your thesis chapters may prove useful.

Thanks for all the helpful responses. I for sure now more about 1916 in the Dominican Republic than anyone in the room. And they have all written phd recs for me. Even though that seems like a landmine. Any phd candidates willing to advise on trying to keep going? i get mixed opinions irl.

Begin with a good night's sleep and dressing for success. Make sure a few of your friends know you'll need some help celebrating later in the day. For your overview, I'd start with a simple statement of your thesis as you understand it ('In my thesis I argue ...'—no real need for an introduction), and then give a quick outline of the evidence you use to support this argument and the interpretations that help you get there. Your committee might benefit from the refresher, and it will open up an opportunity for discussing how your thinking translates into writing.

This is also an opportunity to reflect on the process of writing a thesis, so you might want to consider discussing your reasons for selecting or excluding evidence, referencing models or inspirations that you used for interpreting the evidence, and how this project fits into existing historiography or could be taken in new directions. If appropriate, refer back to some of your coursework that helped in the process.

To prepare for the discussion portion, I'd recommend assessing some of the weaker points in your argument and brainstorming ways they could have been made stronger. Don't let questions about these things surprise you or shake you; academic criticism is given out of love for the craft and a critical reading means that someone is taking your work seriously. Think also about things that went well, such as a choice piece of evidence or a particularly elegant interpretation. Pick one or two that you'll figure out a way to discuss no matter what they ask you; it will remind your readers about the good things in your thesis and give you some firm ground to stand on if you start getting stressed out. And be prepared for the question: 'Would you continue with this as a PhD project?' Think about the potential scope of the project, archival resources, and historiographic significance, but your answer need not be definitive.

Most of all, remember to have fun. The thesis defense is your opportunity to transition from graduate student to professional historian. Three other historians will be in the room, they'll have read your work closely and thought about it carefully. That's a huge complement! They'll be excited to hear from the subject-matter expert and to discuss the things that they've learned. Congratulations on making it this far; knock 'em dead!