The ABC technique for estimating properties of organic compounds from their molecular structures is presented, based on the contributions of Atoms and Bonds to the properties of Conjugates. Conjugates are alternative formal arrangements of valence electrons; a real chemical compound is a hybrid of conjugates. In ABC conjugates of a molecule are generated, and thermodynamic and quantummechanical properties are assigned to each conjugate, by summing contributions from atoms and bonds of the conjugate; the properties of the compound are then obtained as combinations of the properties of all the conjugates. In a simple application, ABC estimates the heat of formation of alkanes within 2.8kJ/mol, compared to 4.7kJ/mol for group contributions. The ultimate goal is the estimation of fractional charges on individual atoms of a compound and electron densities of bonds, because these are related to intermolecular interactions and chemical properties.