Horizontal branch stars are low-mass stars that burn helium in their
cores and hydrogen in a shell surrounding those cores. They play an
important role in a variety of astrophysical contexts, ranging from
our understanding of the interplay between stellar pulsation and
evolution to the formation history of globular clusters and, indeed,
galaxies as a whole. They are also sensitive probes of some key
parameters of particle physics that are poorly constrained by
laboratory experiments. In this talk, I shall discuss several
different problems in which horizontal branch stars do indeed play
an important role, including the early formation history of the Milky
Way and the possibility of multiple populations with widely varying
levels of helium enhancement being present in globular clusters.