Transition State Theory (TST), also known as Activated Complex Theory, provides an important theoretical framework for understanding and qualitatively predicting the rates of chemical reactions. The theory proposes that reactant molecules undergo transformation through a high-energy transition state or activated complex before forming products. The rate of a reaction is strongly related to the Gibbs free energy of activation (ΔG‡), while the enthalpy of activation (ΔH‡) and entropy of activation (ΔS‡) provide additional information about the nature and organization of the transition state. The uploaded article emphasizes the application of these concepts to organic reactions and mixed aqueous–organic solvent systems. It also discusses the Hughes–Ingold approach, hydrogen bonding, ion-solvating power of solvents, and the role of activation parameters in interpreting reaction mechanisms. Solvent polarity, solvation, molecular structure, substituent effects, and temperature can significantly influence the stability of reactants, intermediates, and transition states, thereby modifying reaction rates. The review demonstrates that combining Transition State Theory with solvent effects and activation parameters provides a useful qualitative approach for understanding reaction mechanisms and comparing relative reaction rates.