  Stellar populations contain the most important information about star clus-
ter formation and evolution. Until several decades ago, star clusters were
believed to be ideal laboratories for studies of simple stellar populations
(SSPs). However, discoveries of multiple stellar populations in Galactic
globular clusters have expanded our view on stellar populations in star
clusters. They have simultaneously generated a number of controversies,
particularly as to whether young star clusters may have the same origin as old
globular clusters. In addition, extensive studies have revealed that the SSP
scenario does not seem to hold for some intermediate-age and young star
clusters either, thus making the origin of multiple stellar populations in star
clusters even more complicated. Stellar population anomalies in numerous star
clusters are well-documented, implying that the notion of star clusters as true
SSPs faces serious challenges. In this review, we focus on stellar populations
in massive clusters with different ages. We present the history and progress of
research in this active field, as well as some of the most recent improvements,
including observational results and scenar- ios that have been proposed to
explain the observations. Although our current ability to determine the origin
of multiple stellar populations in star clusters is unsatisfactory, we propose
a number of promising projects that may contribute to a significantly improved
understanding of this subject.
