The TX 2 lamellar dichalcogenides of the transition elements can be regarded as a stacking of two-dimensional [XTX] slabs built of two anionic sheets framing a metallic one. Within a slab, the T element has either octahedral or trigonal prismatic coordination; thus the various structures derived are all related to either the CdI 2 or NbS 2 types. The slabs are bound together by weak forces of the van der Waals type. These forces are weak enough to allow consecutive slabs to slip one on the other: so occur polytypic species that can be classified according to firstly the anionic neighbourhood of the transition element and secondly to the way the slabs stack. Various chemical intercalations can pull the layers apart. Intercalated compounds can be obtained with Lewis base molecules and with various metallic ions, especially with A + alkali metal ions. The A x TX 2 intercalates can be first stage, second stage compounds, etc. In stage I compounds all the van der Waals gaps are occupied, in stage II compounds each alternate gap is occupied. The alkali metal coordination can be either octahedral or trigonal prismatic. It depends on three factors: the size and amount of intercalated atoms, and the nature of the T-X bond within the slabs of the host structure. The structural models and, to a large extent, the phase limits, can be predicted according to an ionicity diagram. Lamellar chalcogenides and their intercalates offer an opportunity for studying physics in two-dimensional systems. Charge density wave studies and superconductivity measurements are of particular interest. The electrochemical applications have rapidly reached the industrial field.