This chapter is brief review of solubility and related concepts, lipid-water partition coefficients and their significance in drug bioavailability. After study of this chapter, students will be able to: • Analyze and predict the solubility of drugs. • Identify factors affecting the solubility of drugs. • Project the polarity, hydrophilicity, hydrophobicity, lipophilicity of drugs and their influence on solubility. • Predict the water or lipid solubility of drugs from their structures. • Comprehend the concept of partition coefficient. • Apply the concept of lipid water partition coefficient (LWPC) and its significance in drug bioavailability and action.