This chapter provides an integrated overview of how hydrostatic pressure regulates the photophysical pathways and photochemical reactions of chromophores through volumetric changes such as molecular conformation and solvation effects. Across a wide range of systems, pressure modifies not only ground-state properties but also excited-state energy landscapes. Pressurization increases solvent density and viscosity and reorganizes solvation shells, while simultaneously stabilizing compact conformers or promoting intramolecular interactions that are inaccessible at ambient conditions. As a result, pressure can either enhance or suppress fluorescence, invert chiroptical signals, or induce ratiometric fluorescence depending on the topology and flexibility of the scaffold. This chapter also highlights how pressure drives or redirects photochemical pathways. In flexible π frameworks, pressure-induced compaction can gate photochemical reactions by favoring stacked conformers. In multiexcitonic systems, pressurization accelerates singlet fission through a transition-state volume contraction while modulating subsequent triplet dynamics via changes in interactions between solvents and chromophores. These case studies reveal that hydrostatic pressure functions as an orthogonal and predictable thermodynamic parameter that directly couples to molecular volume, offering precise control over emissive states, conformational equilibria, and excited-state reactivity.