Crystallization is a phase transformation process where crystals are formed from a source like a cooling saturated solution. Crystallization begins with primary nucleation, which starts from a clear solution without any pre-existing crystals to aid the process. It is divided into homogeneous and heterogeneous types. During heterogeneous nucleation, the crystals form on nucleating surfaces like dust particles. In contrast, homogeneous nucleation takes place in a clear solution without any heterogeneous particulates. During secondary nucleation, the growth occurs due to parent crystals like those introduced through seeding. Nucleating clusters that reach the critical cluster size grow to form larger crystals, whereas those with smaller radii undergo dissolution. Crystal growth occurs when the ratio of concentration and solubility of the solute in the solvent, or the supersaturation ratio, S, exceeds one. Increasing the solute concentration through solvent evaporation or decreasing the solubility of the solute by reducing the temperature increases the value of S. Crystallization is commonly used for manufacturing active pharmaceutical ingredients.