In this article, the NCI method [Johnson et al., J. Am. Chem. Soc., 2010, 132, 6498] for plotting and analysing non-covalent interactions (NCI) is extended to periodic (solid-state) electron densities and implemented in the critic program. The new code uses self-consistent electron densities from a variety of electronic structure methods (pseudopotentials/plane-wave, FP-LAPW, local orbitals, etc.), and it can also build the promolecular density from the crystal geometry alone. As an example of the new code, intermolecular interactions in several molecular crystals are presented and analyzed. The connection with QTAIM studies is established and a reinterpretation of the NCI domains is given regarding the current knowledge of the field. The connection between NCI domains and intermolecular vibrations is made apparent, as well as the ability of the method to reveal the locality of bonding.