Expanded polystyrene (EPS) is an irreplaceable material due to unique properties like lightweight structure, impact tolerant properties and thermal insulation making it essential for daily lives applications. It can be found in applications that take full advantage of these properties such as packing, construction and many others. This chapter explores the chemical composition of EPS, starting from the chemical components of the styrene monomer and its transformation into polystyrene (PS) and EPS. In the process of obtaining EPS, some additives are involved, which are substances that help to obtain EPS with certain characteristics and can also help to improve certain properties that are needed to efficiently use EPS in certain applications. In addition, the path to reach EPS of different densities is explored. EPS foams can be classified based on their density following international norms and standards. The norms ensure the quality of EPS products and define the physical parameters based on their density values. The value of density helps to know what properties each type of EPS foam has, for example, rigidity, mechanical resistance, thermal insulation and resistance to humidity are enhanced when the density values are high. Finally, this chapter highlights the close relationship that exists between the physical properties resulting from each type of EPS, and the apolar nature of styrene and its distribution between the air cells of EPS.