Quantum models with unique ground state are considered. Von Neumann entropy of a part of the ground state [subsystem] is calculated. It shows the entanglement of this part with the rest of the ground state. For a large subsystem asymptotic behavior of the entropy can be related to the gap in the spectrum of the Hamiltonian. For gap-less models the entropy increases logarithmically, for gap-full models it saturates. Several different models will be considered: spin chains, Bose gas, Hubbard model.