Editorial forward (R. Carbó-Doca, P.G. Mezey). Quantum similarity (R. Carbó-Dorca et al.). Fuzzy sets and boolean tagged sets; vector semispaces and convex sets; quantum similarity measures and ASA density functions; diagonal vector spaces and quantum chemistry (R. Carbó-Dorca). Pattern recognition techniques in molecular similarity (W.G. Richards, D. Robinson). Topology and the quantum chemical shape concept (P.G. Mezey). Structural similarity analysis based on topological fragment spectra (Y. Takahashi et al.). Analysis of the transferability of similarity calculations from substructures to complex compounds (G. Sello, M. Termini). Similarity in organic synthesis design: comparing the syntheses of different compounds (G. Sello). Browsable structure-activity datasets (M. Johnson). Characterization of the molecular similarity of chemicals using topological invariants (S.C. Basak et al.). Optimizing hybrid density functionals by means of quantum molecular similarity techniques (M. Solá et al.). Atomic similarity through a neural network: self-associative periodic table of elements (J. Fayos). Comparison of quantum similarity measures derived from one-electron, intracule and extracule densities (X. Fradera et al.). The complementarity principle and its uses in molecular similarity and related aspects (J.R. Dias). Correlations and applications of the circumscribing/excised internal structure concept (J.R. Dias). Least-squares and neural-network forecasting from critical data: diatomic molecular re and triatomic IP and DHa (J. Wholers et al.). Index.