  The star formation triggered in dense walls of expanding shells will be
discussed. The fragmentation process is studied using the linear and non-linear
perturbation theory. The influence of the energy input, the ISM distribution
and the speed of sound is examined analytically and by numerical simulations.
We formulate the condition for the gravitational fragmentation of expanding
shells: if the total surface density of the disc is higher than a certain
critical value, shells are unstable. This value depends on the energy of the
shell and the sound speed in the ISM. As an example the formation of OB
associations near the Sun will be discussed. We trace their orbits in the Milky
Way to see where they have been born: 10 - 12 Myr ago progenitors of
Scorpius-Centaurus OB associations and the Orion OB association resided
together within a sheet-like region elongated in the $l = 20-200degrees
direction, showing that the local OB associations may be formed as fragments of
an expanding supershell.
