  This article discusses density perturbations in inflationary models, offering
a pedagogical description of how these perturbations are generated by quantum
fluctuations in the early universe. A key feature of inflation is that that
rapid expansion can stretch microscopic fluctuations to cosmological
proportions. I discuss also another important conseqence of quantum
fluctuations: the fact that almost all inflationary models become eternal, so
that once inflation starts, it never stops.
