Most are — cleanings, fillings, crowns, root canals, extractions, orthodontics. Purely cosmetic work like whitening is not.
Yes, for treatment that prevents or alleviates dental disease. That covers most of what a dentist does:
Eligible: exams and cleanings, X-rays, fillings, crowns, bridges, root canals, extractions (including wisdom teeth), dentures, implants, periodontal treatment, orthodontics including braces and Invisalign, night guards for bruxism.
Not eligible: teeth whitening and bleaching, veneers done purely for appearance, cosmetic bonding.
The line is medical necessity versus appearance. Braces that correct a bite qualify. Whitening does not.
Orthodontics deserve special attention because payments often stretch across two or three tax years. Keep the contract and the payment records — the treatment plan establishes what you're paying for, and the payment records establish when each expense was incurred.
Dental is one of the three categories tracked here for exactly this reason: it's high-dollar, out-of-pocket, and easy to lose track of.