Mott insulator refers to some condensed-matter systems in which the transport property is greatly influenced by the particle-particle interaction, in contrast with the band insulator, in which the conductivity is greatly influence by the band gap. An example of physical realization of the Mott insulator state can be created with optical lattices, in which neutral Bosonic atoms are trapped. Theoretically, Mott-insulator state can be studied quantitatively with the Hubbard model (for Fermions) or the Bose-Hubbard model (for bosons).