  The microscopic description of the spin-Peierls transition in pure and doped
CuGeO_3 is developed taking into account realistic details of crystal
structure. It it shown that the presence of side-groups (here Ge) strongly
influences superexchange along Cu-O-Cu path, making it antiferromagnetic.
Nearest-neighbour and next-nearest neighbour exchange constants $J_{nn}$ and
$J_{nnn}$ are calculated. Si doping effectively segments the CuO_2-chains
leading to $J_{nn}(Si)\simeq0$ or even slightly ferromagnetic. Strong
sensitivity of the exchange constants to Cu-O-Cu and (Cu-O-Cu)-Ge angles may be
responsible for the spin-Peierls transition itself (``bond-bending mechanism''
of the transition). The nature of excitations in the isolated and coupled
spin-Peierls chains is studied and it is shown that topological excitations
(solitons) play crucial role. Such solitons appear in particular in doped
systems (Cu_{1-x}Zn_xGeO_3, CuGe_{1-x}Si_xO_3) which can explain the
$T_{SP}(x)$ phase diagram.
