Key principles of user interface (UI) design include usability, consistency, visual hierarchy, and accessibility. Usability ensures that interfaces are intuitive, allowing users to navigate easily without extensive training. Consistency across different sections of an application promotes familiarity and reduces cognitive load. Visual hierarchy helps prioritize information, guiding user attention to crucial elements through layout and design choices. Inclusivity is ensured through accessibility, which accommodates users with diverse needs. Adhering to these principles enhances user satisfaction and overall experience with a product.