Until about a decade ago, star clusters were considered "simple" stellar populations: all stars in a cluster were thought to have similar ages and the same chemical composition. Only the individual stellar masses were thought to vary, in essence conforming to a "universal" initial mass function imprinted by their birth processes. Over the past decade, this situation has changed dramatically.
I will discuss my group's exciting recent achievements in this context, with particular emphasis on the properties and the alleged presence of so-called multiple stellar populations in star clusters in the Milky Way and the nearest galaxies across the full age range, from a few tens of millions of years to a few billion years. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate-age (~2 billion-year-old) star clusters, which opens up exciting avenues for future research efforts.