  It has been thirty years since the discovery of the Unruh effect. It has
played a crucial role in our understanding that the particle content of a field
theory is observer dependent. This effect is important in its own right and as
a way to understand the phenomenon of particle emission from black holes and
cosmological horizons. Here, we review the Unruh effect with particular
emphasis to its applications. We also comment on a number of recent
developments and discuss some controversies. Effort is also made to clarify
what seems to be common misconceptions.
