  Open and globular star clusters have served as benchmarks for the study of
stellar evolution due to their supposed nature as simple stellar populations of
the same age and metallicity. After a brief review of some of the pioneering
work that established the importance of imaging stars in these systems, we
focus on several recent studies that have challenged our fundamental picture of
star clusters. These new studies indicate that star clusters can very well
harbour multiple stellar populations, possibly formed through self-enrichment
processes from the first-generation stars that evolved through
post-main-sequence evolutionary phases. Correctly interpreting stellar
evolution in such systems is tied to our understanding of both
chemical-enrichment mechanisms, including stellar mass loss along the giant
branches, and the dynamical state of the cluster. We illustrate recent imaging,
spectroscopic and theoretical studies that have begun to shed new light on the
evolutionary processes that occur within star clusters.
