We investigate chemical reactivity in solution and at aqueous interfaces by combining theory, DFT-based or machine-learning molecular dynamics and enhanced sampling to reveal how solvent structure and dynamics control reaction mechanisms and rates. Building on detailed analyses of solution and interfacial water structure and dynamics, we show how hydrogen-bond rearrangements and electrostatic heterogeneity impact reactivity at aqueous interfaces and in confined environments. Recent applications include acid-base reactions relevant to atmospheric chemistry and how controlling solvation properties at electrochemical interfaces can impact chemical reactivity.