  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.
