Three main concepts: Inheritance which lets programmers structures (classes) that build on some of the attributes of preexisting structures. Helps lets us build on previous work without duplicating work from scratch. Next is encapsulation, which is the practice of keeping attributes within a class private, only providing access to them via public methods. Keeps data within the class safe. Polymorphism, which lets us override or overload methods in a class, depending on the context in which its used.