When basketball games reach a critical moment, what often matters is not whose offense is more flashy, but whose defense is tougher. This sounds like a pet phrase of old coaches, but the data doesn’t lie - in the past five seasons of the NBA Finals, the teams with the top three defensive efficiencies have won four championships. This article doesn’t want to talk about theory in the abstract, but rather starts from real games to discuss how modern basketball defense is built and used. I have sorted out four major systems: man-to-man defense, zone defense, mixed defense, and full-court press, and combined them with impressive defensive plays in the NBA and CBA to analyze how technical details such as rotation and weak-side contraction are executed in actual games. Ultimately, good defense is not about five people standing in a row, but rather a living creature that can breathe, contract, and counterattack.