The document discusses object-oriented (OO) design principles. It defines design smells like rigidity, fragility, immobility, and viscosity that indicate poor design. It then explains principles for good OO design including the single responsibility principle (SRP), open/closed principle (OCP), Liskov substitution principle (LSP), interface segregation principle (ISP), and dependency inversion principle (DIP). SRP states a class should have one responsibility. OCP requires entities be open for extension but closed for modification. LSP defines inheritance and substitutability. ISP avoids fat interfaces. DIP inverts dependencies on abstractions, not concretions.