  The origin of cosmic rays in our Galaxy remains a subject of active debate.
While supernova remnant shocks are often invoked as the sites of acceleration,
it is now widely accepted that the difficulties of such sources in reaching PeV
energies are daunting and it seems likely that only a subclass of rare remnants
can satisfy the necessary conditions. Moreover the spectra of cosmic rays
escaping the remnants have a complex shape that is not obviously the same as
the spectra observed at the Earth. Here we investigate the process of particle
acceleration at the termination shock that develops in the bubble excavated by
star clusters' winds in the interstellar medium. While the main limitation to
the maximum energy in supernova remnants comes from the need for effective wave
excitation upstream so as to confine particles in the near-shock region and
speed up the acceleration process, at the termination shock of star clusters
the confinement of particles upstream in guaranteed by the geometry of the
problem. We develop a theory of diffusive shock acceleration at such shock and
we find that the maximum energy may reach the PeV region for powerful clusters
in the high end of the luminosity tail for these sources. A crucial role in
this problem is played by the dissipation of energy in the wind to magnetic
perturbations. Under reasonable conditions the spectrum of the accelerated
particles has a power law shape with a slope $4\div 4.3$, in agreement with
what is required based upon standard models of cosmic ray transport in the
Galaxy.
