Faced with a huge number of descriptions that allow the characterization of a molecular structure, it may be useful to classify the molecular descriptors according to the nature of the encoded information and their dimensionality. Remember that such descriptors correspond to scalar quantities (or small vectors) condensing the information (structural and/or chemical) associated with the molecular formula. A fruitful advance in the eld was to consider the structural formula of a molecule as a graph, where vertices and edges correspond to atoms and bonds. This opened the way to the denition of a large number of indices, later extended to 3D structures. On the other hand, diverse encoding schemes were proposed to express the chemical information in a numerical form, which was to be the input in a statistical package.