EPR spectroscopy can map out the electron distribution in a molecule, such the same way as proton NMR spectroscopy can map out the proton distribution, and it provides some of the most direct evidence for the principal concepts underlying the electronic theory of organic structure and mechanism. This is illustrated for the phenomena of conjugation, hyperconjugation, substituent effects in annulenes, Hückel theory, ring strain, the Mills-Nixon effect, and ion pairing.