Robert M. Wald reviews the current state of black hole thermodynamics, covering classical black hole thermodynamics, Hawking radiation, the generalized second law (GSL), and entropy bounds. He discusses the derivation of black hole thermodynamics, the status of Hawking radiation, the GSL, and the issue of black hole entropy. He also addresses unresolved issues in black hole thermodynamics, such as the black hole information paradox and the nature of black hole entropy. Black hole thermodynamics reveals a deep connection between gravity, thermodynamics, and quantum theory. The laws of black hole mechanics resemble the laws of thermodynamics, with the area of a black hole's event horizon behaving like entropy. Hawking radiation, a quantum effect, shows that black holes emit thermal radiation, allowing the laws of black hole mechanics to be interpreted as thermodynamic laws. The GSL states that the total entropy (ordinary entropy plus black hole entropy) never decreases, providing strong evidence that the area of a black hole is its entropy. Classical black hole thermodynamics includes the zeroth and first laws, which are analogous to thermodynamic laws. The zeroth law states that the surface gravity of a black hole is constant over its event horizon, while the first law relates changes in mass, angular momentum, and horizon area. The second law, analogous to the second law of thermodynamics, states that the total entropy of the universe never decreases. Hawking radiation, discovered in 1974, shows that black holes emit thermal radiation with a temperature proportional to their surface gravity. This radiation allows the GSL to hold, as the entropy of the black hole increases while the entropy of the surrounding matter decreases. The GSL is supported by arguments that show that the total entropy (ordinary entropy plus black hole entropy) never decreases. Entropy bounds are also discussed, with Bekenstein proposing a universal bound on the entropy-to-energy ratio of bounded matter. This bound is necessary for the validity of the GSL, but recent arguments suggest that it may not be required. The GSL is supported by arguments that show that the total entropy (ordinary entropy plus black hole entropy) never decreases, even in the presence of quantum effects. The GSL remains a central issue in black hole thermodynamics, with ongoing research into its validity and implications.Robert M. Wald reviews the current state of black hole thermodynamics, covering classical black hole thermodynamics, Hawking radiation, the generalized second law (GSL), and entropy bounds. He discusses the derivation of black hole thermodynamics, the status of Hawking radiation, the GSL, and the issue of black hole entropy. He also addresses unresolved issues in black hole thermodynamics, such as the black hole information paradox and the nature of black hole entropy. Black hole thermodynamics reveals a deep connection between gravity, thermodynamics, and quantum theory. The laws of black hole mechanics resemble the laws of thermodynamics, with the area of a black hole's event horizon behaving like entropy. Hawking radiation, a quantum effect, shows that black holes emit thermal radiation, allowing the laws of black hole mechanics to be interpreted as thermodynamic laws. The GSL states that the total entropy (ordinary entropy plus black hole entropy) never decreases, providing strong evidence that the area of a black hole is its entropy. Classical black hole thermodynamics includes the zeroth and first laws, which are analogous to thermodynamic laws. The zeroth law states that the surface gravity of a black hole is constant over its event horizon, while the first law relates changes in mass, angular momentum, and horizon area. The second law, analogous to the second law of thermodynamics, states that the total entropy of the universe never decreases. Hawking radiation, discovered in 1974, shows that black holes emit thermal radiation with a temperature proportional to their surface gravity. This radiation allows the GSL to hold, as the entropy of the black hole increases while the entropy of the surrounding matter decreases. The GSL is supported by arguments that show that the total entropy (ordinary entropy plus black hole entropy) never decreases. Entropy bounds are also discussed, with Bekenstein proposing a universal bound on the entropy-to-energy ratio of bounded matter. This bound is necessary for the validity of the GSL, but recent arguments suggest that it may not be required. The GSL is supported by arguments that show that the total entropy (ordinary entropy plus black hole entropy) never decreases, even in the presence of quantum effects. The GSL remains a central issue in black hole thermodynamics, with ongoing research into its validity and implications.