This reprint presents a comprehensive review of fundamental principles in an easily understandable manner. It deals with the fundamental properties of drug substances such as solubility, stability, surface & interfacial phenomena, rheology, micromeritics, & complexation which will give a lead in formulating drug substances into suitable dosage forms. In addition, it includes ICH guidelines for stability testing and also suggestions for practicals wherever necessary. Moreover, the language is so simple that gurantees more clarity than brevity, We do hope the presentation will motivate self study.