Solvation free energy is one of the key quantities in chemistry and biology as most of the molecular phenomena occur in different solvents. Water being the most versatile solvent, determination or calculation of hydration free energy (HFE) is the most important step in understanding any complex process. The calculation of HFE is typically done using either a quantum mechanical description of the solute in a dielectric continuum or a classical description of the solute where water is treated either with explicit models or implicit models (which are mostly dielectric continuum). The classical forcefields are usually used for macromolecules, and for small molecules interacting with macromolecules, classical forcefields are used for compatibility. Henceforth, all discussions on the calculation of hydration free energy will be with the classical models. Although the physics-based methods are well developed, there are outstanding issues in getting accurate solvation free energy and it has been an active area of new developments and improvements. The most rigorous calculations are alchemical methods like thermodynamic integration and free energy perturbation. However, these calculations are time-consuming and generally speaking, are not suitable for a large set of molecules.