The CRYSTAL package performs ab-initio calculations on the ground-state energy, electronic wave function and properties of periodic systems. Hartree-Fock and Kohn-Sham Hamiltonians (DFT) can be used. Systems may be periodic in 0 (molecules), 1 (polymers), 2 (slabs) or 3 dimensions (crystals). Single-particle wave functions are expressed in terms of linear combinations of so-called Bloch orbitals. The program was designed by V. R. Saunders, R. Dovesi, C. Roetti, M. Causa, N. M. Harrison, and C. M. Zicovich-Wilson. It is the only available solid-state code that is able to perform Hartree-Fock type calculations in a tight-binding model.