Coronins are highly conserved regulators of the actin cytoskeleton whose structure and biological function have remained mysterious until recently. They were originally identified in Dictyostelium, where they localize to actin-rich crown-like structures on the dorsal surface of cells. Coronins bind filamentous actin and the Arp2/3 complex and are involved in modulating actin dynamics. Unlike other known Arp2/3-binding proteins, coronins inhibit Arp2/3 nucleating activity. Genetic data from Dictyostelium, yeast and Drosophila indicate that coronins are important regulators of several actin-dependent physiological processes. Here, we review recent insights into mammalian coronin structure, function and regulation and identify key questions that remain unanswered in this field.