The key principles of user interface (UI) design include consistency, simplicity, and feedback. Consistency ensures that design elements serve a unified purpose, helping users predict how the interface will respond. Simplicity involves creating intuitive layouts that minimize unnecessary complexity, enabling users to navigate easily. Feedback gives users information about their actions, reassuring them that the system is responsive and functional. These principles are essential in creating user-friendly interfaces that enhance usability and overall user satisfaction, ultimately leading to a more successful product.