  The microscopic description of spin-Peierls substances is discussed.
Particular attention is paid to the ordered (dimerised and incommensurably
modulated) phases. Important points are the adiabatic and the antiadiabatic
approach, generic soliton forms, elastic and magnetic interchain couplings. The
wealth and the accuracy of experimental information collected for the first
inorganic spin-Peierls substance CuGeO_3 motivates this work.
