This chapter introduces the reader to critical physical properties of water (i.e., high dielectric constant, low density, high melting point, high boiling point, high heat capacity, interesting temperature-density relationship) that make it a highly unique substance in the universe. Section 3.4 presenting the Steam Tables is unquestionably the most often referenced data for all engineering disciplines and can justifiably be considered the key element in this chapter. An application section provides six Illustrative Examples related to the general subject of water’s physical properties.