This chapter explores the behavior of dielectric materials (insulators) in electric fields, contrasting them with conductors. It introduces the polarization vector ( \(\textbf{P}\) ) as a macroscopic quantity representing the average electric dipole moment density within a medium, which arises from the distortion or alignment of molecular dipoles. The chapter demonstrates how a polarized dielectric can be effectively modeled as a distribution of bound charges (volume and surface densities) that contribute to the total electric field. A key concept introduced is the electric displacement field ( \(\textbf{D}\) ), defined as \(\textbf{D}=\epsilon _0 \textbf{E}+\textbf{P}\) . The chapter shows that sources for \(\textbf{D}\) are solely free charges, simplifying Gauss’s law in dielectric media. It examines linear, homogeneous, and isotropic (LHI) media, where \(\textbf{P}\) is linearly related to \(\textbf{E}\) , leading to the definition of electric susceptibility ( \(\chi \) ) and the dielectric constant ( \(\epsilon _r\) ). The physical interpretation of \(\epsilon _r\) is emphasized: it quantifies the reduction of the electric field within the dielectric due to the screening effect of bound charges. The chapter also discusses various polarization mechanisms, including electronic, ionic, and orientational polarization, and derives the Clausius–Mossotti relation, which connects microscopic polarizability to the macroscopic dielectric constant. Crucially, it establishes the boundary conditions for electric fields and displacement fields at the interface between different dielectric media. Finally, the chapter examines the impact of dielectrics on capacitance, demonstrating how inserting a dielectric material increases the ability of a capacitor to store charge. It also introduce the electrostatic free energy for polarized media and the forces and torques exerted on dielectrics in electric fields, providing a comprehensive understanding of their macroscopic behavior.