The paper by B. L. Hu provides an overview of the current research in stochastic semiclassical gravity, a theory that generalizes the semiclassical Einstein equation to an Einstein-Langevin equation with a stochastic source term arising from the fluctuations of the energy-momentum tensor of quantum fields. The author discusses recent applications of this theory to black hole fluctuation and backreaction problems, linear response of hot flat space, and structure formation in inflationary cosmology. He emphasizes the importance of viewing semiclassical gravity as mesoscopic physics and general relativity as the hydrodynamic limit of spacetime quantum substructures. The paper explores three basic issues—stochasticity, collectivity, and correlations—and three processes—dissipation, fluctuations, and decoherence. It also discusses methods for probing high-energy activity from below, such as effective field theory and the correlation hierarchy. The author highlights the significance of stochastic behavior at low energy and correlation noise in carrying information about the high-energy sector. Finally, he describes processes at the Planck scale, including tunneling, pair creation, wave scattering in random geometry, and spacetime foams.The paper by B. L. Hu provides an overview of the current research in stochastic semiclassical gravity, a theory that generalizes the semiclassical Einstein equation to an Einstein-Langevin equation with a stochastic source term arising from the fluctuations of the energy-momentum tensor of quantum fields. The author discusses recent applications of this theory to black hole fluctuation and backreaction problems, linear response of hot flat space, and structure formation in inflationary cosmology. He emphasizes the importance of viewing semiclassical gravity as mesoscopic physics and general relativity as the hydrodynamic limit of spacetime quantum substructures. The paper explores three basic issues—stochasticity, collectivity, and correlations—and three processes—dissipation, fluctuations, and decoherence. It also discusses methods for probing high-energy activity from below, such as effective field theory and the correlation hierarchy. The author highlights the significance of stochastic behavior at low energy and correlation noise in carrying information about the high-energy sector. Finally, he describes processes at the Planck scale, including tunneling, pair creation, wave scattering in random geometry, and spacetime foams.