This chapter focuses on the physical properties of seawater. Many of the unique characteristics of the ocean can be ascribed to the nature of water itself. Consisting of two positively charged hydrogen ions and a single negatively charged oxygen ion, water is arranged as a polar molecule having positive and negative sides. This molecular polarity leads to water's high dielectric constant (ability to withstand or balance an electric field). Water is able to dissolve many substances because the polar water molecules align to shield each ion, resisting the recombination of the ions. As seawater is heated, molecular activity increases and thermal expansion occurs, reducing the density. Another important physical characteristic of seawater is its temperature. In most of the ocean, temperature is the primary determinant of density. The chapter discusses molecular properties of water, and thermal properties of seawater. Effects of temperature and salinity on density, and concepts related to sound and light in the sea are also explained in the chapter.