Controlling chemical reactions with precision is a central objective across chemistry, materials science, and biology. While thermally driven reactions are well described by classical thermodynamics and catalyst design, photochemical processes remain considerably more difficult to predict because excited states evolve on ultrafast timescales and are governed by weak intermolecular interactions. As a result, their reactivity is highly sensitive to the balance between enthalpic and entropic contributions, and temperature often fails as a reliable control variable due to enthalpy-entropy compensation and increased nonradiative decay at elevated temperatures. These limitations highlight the need for an alternative thermodynamic parameter capable of modulating reaction pathways independently of entropy. Hydrostatic pressure provides such a variable by coupling directly to reaction volume changes associated with conformational rearrangements, solvation, and molecular organization. Early studies in molecular, supramolecular, and polymer systems revealed pronounced pressure effects on excimer formation, charge transfer (CT) states, and host-guest dynamics, yet only recently have these phenomena been unified within a coherent framework based on reaction volume changes. Extending this concept to biological environments has further strengthened the connection between photochemistry and mechanobiology, enabling the development of novel chemosensors that probe how cells respond to mechanical stimuli. This chapter outlines these unified thermodynamic principles and their implications for reaction control across systems ranging from individual molecules to living systems.