The chapter discusses the quasinormal modes (QNMs) of black holes, which are oscillations that occur when a black hole is perturbed. These modes are crucial in understanding the behavior of black holes, particularly in the context of astrophysics and string theory. The authors review various methods for calculating QNMs, including numerical and analytical techniques, and highlight the importance of these modes in predicting the behavior of quark-gluon plasmas in strongly coupled field theories. They also discuss the stability of black holes, the late-time tails of gravitational waves, and the connection between QNMs and holographic superconductors. The chapter emphasizes the significance of QNMs in both astrophysical observations and theoretical studies, such as the AdS/CFT correspondence, and provides a comprehensive overview of the current state of research in this field.The chapter discusses the quasinormal modes (QNMs) of black holes, which are oscillations that occur when a black hole is perturbed. These modes are crucial in understanding the behavior of black holes, particularly in the context of astrophysics and string theory. The authors review various methods for calculating QNMs, including numerical and analytical techniques, and highlight the importance of these modes in predicting the behavior of quark-gluon plasmas in strongly coupled field theories. They also discuss the stability of black holes, the late-time tails of gravitational waves, and the connection between QNMs and holographic superconductors. The chapter emphasizes the significance of QNMs in both astrophysical observations and theoretical studies, such as the AdS/CFT correspondence, and provides a comprehensive overview of the current state of research in this field.