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Features of the electron density distribution in the ground and excited states, the position of the energy levels, and the equilibrium molecular geometry of the ions of linear p-electron systems, including conjugated polymers and oligomers, cationic and anionic polymethine dyes, and the radical-ions of a,w-substituted polyenes, are reviewed. By using the soliton concept it was possible to explain many unusual properties of this type of organic compound: The quasimetallic conductivity, the marked change in the spectral characteristics of ionic dyes absorbing and emitting light in the near IR region of the spectrum, etc. It was shown on the basis of semiempirical and nonempirical calculations that the charges in collective p-electronic systems form soliton waves, the width of which does not depend on the length of the molecule. In the excited state the size of the solitons increases significantly. The introduction of donor and acceptor terminal groups can destroy the symmetry of the charge distribution and molecular geometry at a conjugation chain of critical length exceeding the dimensions of the soliton. It was established that the injection of electrons/holes not only leads to the appearance of a soliton level at the center of the energy gap but is also accompanied by significant displacement of the top of the valence band and the bottom of the conduction band.