  The existence of complex stellar populations in some star clusters challenges
the understanding of star formation. E.g. the ONC or the sigma Orionis cluster
host much older stars than the main bulk of the young stars. Massive star
clusters (omega Cen, G1, M54) show metallicity spreads corresponding to
different stellar populations with large age gaps. We show that (i) during star
cluster formation field stars can be captured and (ii) very massive globular
clusters can accrete gas from a long-term embedding inter stellar medium and
restart star formation.
