This chapter is intended to explain some theoretical aspects of interfaces in polymer nanocomposites. Interfaces are considered to play a crucial role in determining the performances of polymer nanocomposites. Interfaces between inorganic fillers and polymer matrices have long been investigated in colloid science, since the 1930s, and then analyzed with modern quantum mechanics in 1950s. Bonding between two immiscible substances, i.e., organic polymers and inorganic fillers, are needed for homogeneous filler dispersion and the formation of thermodynamically stable composites. Bound polymers were the first concept for interfaces clarified by colloid chemistry. Many factors such as triboelectricity, electrochemical potential, Coulombic attractive force, steric effects, and quantum mechanical forces are involved in the process of determining polymer morphology and the formation of nanocomposites via mutual interactions.