This chapter is divided into three parts. The first part deals with the interrelation of tonicity, osmotic pressure, osmolarity, osmosis, passive transmembrane diffusion, and drug absorption and distribution. The second part discusses the preparation of isotonic solutions. The third part describes the tonicity adjustment of hypotonic and hypertonic solutions. A thermodynamic approach is used to rationalize the preparation of isotonic solutions and conversion of hypotonic or hypertonic solutions to isotonic. The application of the formulas is demonstrated step-by-step through many examples extracted from physiology and pharmaceutics. An extensive justification of the importance of isotonic solutions in pharmacy is provided to motivate you to read the chapter carefully and solve all the practice problems. The equations are not just cited in the book, but they are rather derived and explained creatively. In addition, concepts like osmolarity and osmotic pressure are explained from the biophysical and physiological point of view, thus offering to you a better understanding of the phenomenon. Many of the problems are unique in the sense that you learn to calculate osmolarity of a drug solution solely from the chemical structure of the compound. Lastly, adjusting the osmolarity of hypertonic solutions is a new concept presented in this book.