1. Introduction; 2. Mental incapacity; 3. Non est factum in English law; 4. Common mistake in English law; 5. Mistake as to identity in English law; 6. Mistake as to terms in English law; 7. Mutual mistake in English law; 8. Mistake under the PICC, PECL, DCFR and US Restatement (Second) of Contracts; 9. Misrepresentation; 10. Duress; 11. Undue influence; 12. Unconscionability; 13. Undue influence and unconscionability under the PICC, PECL and DCFR; 14. Conclusion.