The SOLID principles are a set of five design principles in object-oriented programming that help developers write code that is more maintainable, scalable, and easier to understand. These principles were introduced by Robert C. Martin, also known as "Uncle Bob".

1. S - Single Responsibility Principle (SRP).
2. O - Open/Closed Principle (OCP).
3. L - Liskov Substitution Principle (LSP).
4. I - Interface Segregation Principle (ISP).
5. D - Dependency Inversion Principle (DIP).

These principles are fundamental in designing robust, flexible, and maintainable software systems. Following SOLID principles helps in avoiding common issues like tightly coupled code, code duplication, and a lack of scalability.