Inflation and Eternal Inflation

The basic workings of inflationary models are summarized, along with thearguments that strongly suggest that our universe is the product of inflation.The mechanisms that lead to eternal inflation in both new and chaotic modelsare described. Although the infinity of pocket universes produced by eternalinflation are unobservable, it is argued that eternal inflation has realconsequences in terms of the way that predictions are extracted fromtheoretical models. The ambiguities in defining probabilities in eternallyinflating spacetimes are reviewed, with emphasis on the youngness paradox thatresults from a synchronous gauge regularization technique. Vilenkin's proposalfor avoiding these problems is also discussed