2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/210982It 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.53 pages, 11 figures, submitted to Reviews of Modern PhysicsGeneral Relativity and Quantum CosmologyHigh Energy Physics - Theorytext