The Physical Origin of the Chemical Bond.- Absolute Electronegativity and Absolute Hardness.- Valence in Molecular Orbital Theory.- The Magnetic Description of Conjugated Hydrocarbons.- Directional Properties of Covalent Bonding in Molecules.- The Two-Electron Bond as a Molecular Building Block.- Interpretation of Molecular Behaviour by Localized Molecular Orbitals (LMOs).- Properties of the Fermi Hole and Electronic Localization.- The Diatomics-in-Molecules Method and the Chemical Bond.- Calculation of Electron Correlations by Using Local Operators.- The Concept of the Chemical Bond in Solids.- Chemical Implication of Local Features of the Electron Density Distribution.- Electron Deformation Densities and Chemical Bonding in Transition Metal Complexes.- Fundamentals of Relativistic Effects in Chemistry.