Globular clusters as laboratories for stellar evolution

Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and—crucially- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium-abundance indicators, and present an overview of available constraints on the helium abundance in globular clusters.