Freeze drying or lyophilization of aqueous solutions is widely used in pharmaceutical industry. The increased importance of the process is gaining a worldwide interest of research. A growing body of literature has demonstrated that the scientific approach can result in improved product quality with minimum trial and error empiricism. Formulation and process development need a systematical understanding of the physical chemistry of freezing and freeze drying, material science and mechanisms of heat and mass transfer. This paper presents an overview on freeze drying of aqueous solutions based on publications in the past few decades. The important issues of the process are analyzed.