Molecular connectivity indices are descriptors of the molecular accessibility. The first- and second-order connectivity indices represent molecular accessibility surface areas and volumes, and higher order indices represent magnitudes in higher dimensional spaces. The essence of molecular connectivity is the encoding of a molecular structure in a non-empirical way. It is not a measured characteristic, and it does not derive from a particular physical property, and it does not translate into one. Kier and Hall developed a valence molecular connectivity index, derived from the non-hydrogen part of the molecule. So this leads to a first classification of the molecular connectivity indices: simple molecular connectivity indices and valence molecular connectivity indices. The cluster and path-cluster indices are describing local structural properties, for example, the branching degree in a molecule. They are highly sensitive to changes in branching and their value rapidly increases with the degree of branching.